Makefile.am: fix parallel build of t/generated-code2/cxx-generate-packed-data

The object file name is not what is expected when building with
automake's "subdir-objects" option. Rather than expecting automake to
generate a specific filename, we simply make the generated header a
dependency of all objects for cxx-generate-packed-data.

(Issue #156, #169.)
build-cmake/: add a new CMakeLists.txt file as a fallback

For certain platforms where autotools is not the preferred build system,
provide a fallback cmake file that can compile protoc-c and a static
library of libprotobuf-c.

Based on the file from alex85k's protobuf-c repository.

(Issue #168.)
Makefile.am: distribute build-cmake/CMakeLists.txt

Ironically, we need to tell automake to distribute the CMakeLists.txt
file in the tarball...