fix VPATH builds

this commit fixes VPATH builds where the build tree is separate from the
source tree. this will allow us to run "make distcheck".
configure.ac: print out C++ related build flags
build system: remove --disable-protoc option

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.
configure.ac: add the protobuf version to the configure summary
Makefile.am: protobuf-c-rpc/: make sure that the gsk* header files get distributed
.travis.yml: use system protobuf packages and build using distcheck

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".