it's possible for the <google/protobuf/compiler/> header files to be
shipped in a separate package (e.g., debian's libprotoc-dev). check for
this at configure time rather than allowing the build process to fail.

there is some confusion with regard to the use of lower case letters in
enum values. take the following message definition:

    message LowerCase {
      enum CaseEnum {
        UPPER = 1;
        lower = 2;
      }
      optional CaseEnum value = 1 [default = lower];
    }

this generates the following C enum:

    typedef enum _LowerCase__CaseEnum {
      LOWER_CASE__CASE_ENUM__UPPER = 1,
      LOWER_CASE__CASE_ENUM__lower = 2
        _PROTOBUF_C_FORCE_ENUM_TO_BE_INT_SIZE(LOWER_CASE__CASE_ENUM)
    } LowerCase__CaseEnum;

note that the case of the enum value 'lower' was preserved in the C
symbol name as 'LOWER_CASE__CASE_ENUM__lower', but that the _INIT macro
references the same enum value with the (non-existent) C symbol name
'LOWER_CASE__CASE_ENUM__LOWER':

    #define LOWER_CASE__INIT \
     { PROTOBUF_C_MESSAGE_INIT (&lower_case__descriptor) \
        , 0,LOWER_CASE__CASE_ENUM__LOWER }

additionally, the ProtobufCEnumValue array generated also refers to the
same enum value with the (non-existent) upper cased version:

    const ProtobufCEnumValue lower_case__case_enum__enum_values_by_number[2] =
    {
      { "UPPER", "LOWER_CASE__CASE_ENUM__UPPER", 1 },
      { "lower", "LOWER_CASE__CASE_ENUM__LOWER", 2 },
    };

we should preserve the existing behavior of copying the case from the
enum values in the message definition and fix up the places where the
(non-existent) upper case version is used, rather than changing the enum
definition itself to match the case used in the _INIT macro and
enum_values_by_number array, because it's possible that there might be
existing working code that uses enum values with lower case letters that
would be affected by such a change.

incidentally, google's C++ protobuf implementation preserves case in
enum values. protoc --cpp_out generates the following enum declaration
for the message descriptor above:

    enum LowerCase_CaseEnum {
      LowerCase_CaseEnum_UPPER = 1,
      LowerCase_CaseEnum_lower = 2
    };

this function is now static, so it shouldn't have the "protobuf_c_"
prefix in its name.

Also, don't default dot support to on and emit something to say if
the dogygen-doc target is enabled.

Still need to add the comments in the source code. Currently I've
seeded it with the libprotobuf-c files.  I've configured it
to make man pages and html pages.  Might not be ideal, but makes it easy
for me to check things (html is nicer, but man pages are handier for
remote servers).

Use MKDIR_P and LN_S rather than their usual expansions.

This reverts commit aaa40c58.

of course, protobuf-c uses both C and C++.

    It’s important to note that, differently from what we’ve seen for
    the serial test harness (see Parallel Test Harness), the
    AM_TESTS_ENVIRONMENT and TESTS_ENVIRONMENT variables cannot be use
    to define a custom test runner; the LOG_COMPILER and LOG_FLAGS (or
    their extension-specific counterparts) should be used instead:

    ## This is WRONG!
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib' $(PERL) -Mstrict -w

    ## Do this instead.
    AM_TESTS_ENVIRONMENT = PERL5LIB='$(srcdir)/lib'; export PERL5LIB;
    LOG_COMPILER = $(PERL)
    AM_LOG_FLAGS = -Mstrict -w

(http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html)

"As with the serial harness above, by default one status line is printed
per completed test, and a short summary after the suite has completed.
However, standard output and standard error of the test are redirected
to a per-test log file, so that parallel execution does not produce
intermingled output. The output from failed tests is collected in the
test-suite.log file. If the variable ‘VERBOSE’ is set, this file is
output after the summary."

(http://www.gnu.org/software/automake/manual/html_node/Parallel-Test-Harness.html)

Makefile.am: add valgrind to the AM_TESTS_ENVIRONMENT
configure.ac: enable valgrind testing option for ./configure
m4/valgrind-tests.m4: enable tracing children for libtool wrapper
script compatibility, but ignore standard binaries in /usr or /bin

it turned out to be a bad idea to not export the default allocator's
methods via the ProtobufCAllocator's function pointers. there is at
least one user (protobuf-c-rpc) that makes allocations by directly
invoking those methods.

use a linker script rather than relying on libtool's
-export-symbols-regex. this lets us start versioning the library's
exported symbols.

in particular we no longer use abort().

according to microsoft's platform evangelist, "we recommend that you
consider using a different compiler such as Intel or gcc" if you need a
conforming C compiler. since there's already a project that maintains
the stdint.h / inttypes.h headers for microsoft compilers
(https://code.google.com/p/msinttypes/) there's not much point in
maintaining this ourselves.

there's not much point to having the "private" definitions split out
into a separate header file. they're still in the namespace and there's
nothing that can be done to prevent "unauthorized" uses. just integrate
the definitions into the main header file but put them in the bottom and
note that they're "private".

this makes it very slightly easier to copy the protobuf-c support
library into another project wholesale, since one less file is required.

i'm not quite sure what this thing is used for or how useful it could
possibly be. it's not used anywhere in protobuf-c or protobuf-c-rpc, and
it wasn't in protobuf-c 0.14 or earlier. just delete it; if anything
actually needs this kind of logic, it's easy enough to replicate it.

libraries should never generate output on their own to stdout/stderr.
remove the PRINT_UNPACK_ERRORS macro and rename UNPACK_ERROR to
PROTOBUF_C_UNPACK_ERROR.

the error strings are left in but compiled out by default. they could
theoretically be re-enabled for a debugging session by changing the
PROTOBUF_C_UNPACK_ERROR macro to something like:

	#define PROTOBUF_C_UNPACK_ERROR(...) do { fprintf(stderr, __VA_ARGS__); fputc('\n', stderr); } while (0)

it might be premature to remove the <google/> prefix from the include
path. set up a compatibility symlink so that old code continues to
compile.

some of these headers aren't used in the protobuf-c code base any more,
and in any case the results of these checks (the HAVE_*_H defines in
config.h) are not actually used anywhere and the absence of any of these
headers doesn't cause configure to fail, so just delete these useless
checks.

protobuf-c: replace DO_ALLOC, FREE, system_alloc, system_free with inline memory allocation functions

this reworks memory allocation throughout the support library.

the old DO_ALLOC macro had several problems:

  1) only by reading the macro implementation is it possible to tell
  what actually occurs. consider:

        DO_ALLOC(x, ...);

  vs.:

        x = do_alloc(...);

  only in the latter is it clear that x is being assigned to.

  2) it looks like a typical macro/function call, except it alters the
  control flow, usually by return'ing or executing a goto in the
  enclosing function. this type of anti-pattern is explicitly called out
  in the linux kernel coding style.

  3) in one instance, setting the destination pointer to NULL is
  actually a *success* return. in parse_required_member(), when parsing
  a PROTOBUF_C_TYPE_BYTES wire field, it is possible that the field is
  present but of zero length, in which case memory shouldn't be
  allocated and nothing should actually be copied. this is not apparent
  from reading:

        DO_ALLOC(bd->data, allocator, len - pref_len, return 0);
        memcpy(bd->data, data + pref_len, len - pref_len);

  instead, make this behavior explicit:

        if (len - pref_len > 0) {
                bd->data = do_alloc(allocator, len - pref_len);
                if (bd->data == NULL)
                        return 0;
                memcpy(bd->data, data + pref_len, len - pref_len);
        }

  this is much more readable and makes it possible to write a
  replacement for DO_ALLOC which returns NULL on failures.

this changes the protobuf_c_default_allocator to contain only NULL
values; if a replacement function pointer is not present (non-NULL) in
this struct, the default malloc/free implementations are used. this
makes it impossible to call the default allocator functions directly and
represents an API/ABI break, which required a fix to the
PROTOBUF_C_BUFFER_SIMPLE_CLEAR macro.

despite turning one-line allocations in the simple case:

	DO_ALLOC(rv, allocator, desc->sizeof_message, return NULL);

into three-line statements like:

	rv = do_alloc(allocator, desc->sizeof_message);
	if (!rv)
		return (NULL);

this changeset actually *reduces* the total number of lines in the
support library.