PACKAGE_STRING provided by autotools (fixes #52)

in case the client has failed to connect and got destroyed (fixes #70)

protobuf-c-rpc/t/test-rpc.c: add a test case for #70

messages on the client side (fixes #78)

protobuf-c-rpc/t/test-rpc.c: add a test case for #78

per https://code.google.com/p/protobuf/source/detail?r=50, the license
on google-originated protobuf code was switched from Apache-2.0 to
BSD-3-Clause (so-called "New BSD" license). update any of the google
license statements to use this new license.

per email with dave, drop the third clause on his BSD-3-Clause license,
so this now becomes BSD-2-Clause.

make sure to use consistent indentation and wrapping throughout.

the change that required protobuf >= 2.5.0 has been reverted, so there's
no need to build protobuf from source. use the system protobuf packages
instead.

also use distcheck to build and check the distribution. this will run
the test suite and perform various other checks. it will also perform a
VPATH build using the distribution tarball, which will catch any files
present in the git repository but inadvertently not distributed in the
tarball.

also test the build when configured with "--enable-rpc".

for some reason, "make distcheck" fails when the build is configured
with --disable-protoc. i haven't been able to track down the root cause
(it appears the build is trying to generate the generated pb-c files for
the test cases, but the test cases should be disabled when
--disable-protoc is specified). so, remove the --disable-protoc option.
libprotobuf-c should always be tested as part of a build, and this can't
be done unless protoc-c is built. this also reduces the number of build
combinations that need to be tested.

this commit fixes VPATH builds where the build tree is separate from the
source tree. this will allow us to run "make distcheck".

this option is only supported by the upstream protobuf compiler starting
with version 2.5.0. this version is not yet widely available in the
debian/ubuntu repositories, and we would like to avoid breaking the
build on those platforms with the distribution version of protobuf
installed, so revert the following commits:

 - 5ee9c034
 - 84e41e73

as far as i can tell, this code is not used anywhere.