fixed gcc warnings 'integer constant is so large that it is unsigned' for large 64 bit constants (default values)

protoc may not be on the default PATH, so augment $PATH with the
executable path registered by pkg-config for the protobuf package.

additionally declare PROTOC as a precious variable, thus allowing it to
be explicitly set by the user at ./configure time.

based on a patch from Andrei Nigmatulin.

the protobuf header files may be installed in a non-standard location
and thus we need to use the CFLAGS registered for protobuf in pkg-config
in order to find them.

based on a patch from Andrei Nigmatulin.

Makefile.am: use CXXFLAGS instead of CFLAGS for protoc-c/protoc-c, t/generated_code2_cxx_generate_packed_data

these are C++ programs, not C programs, so use CXXFLAGS instead of
CFLAGS.

based on a patch from Andrei Nigmatulin.

if pkg-config is installed, the libprotobuf-c .pc file will be
installed; if pkg-config is not installed, the .pc file won't be
installed.

this behavior only applies when we're building with ./configure
--disable-protoc, since pkg-config is required in order to detect the
protobuf dependency.

this is conditional on whether the linker supports version scripts, for
which we use the gl_LD_VERSION_SCRIPT macro from the gnulib project.

on platforms without version scripts, we fall back to libtool's
-export-symbols-regex.

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