src/Makefile.am: Include .c and .h files generated from Protocol Buffers in the distribution.

Fixes Github issue #269.

Signed-off-by: Florian Forster <octo@collectd.org>
src/Makefile.am: Move *.pb-c.[ch] to MAINTAINERCLEANFILES.

Only generate build rules for these files if protoc-c is available and
use the provided files from the distribution if it is not.
configure.in: Fix dependency of the pinba and write_riemann plugins.

Since the generated *.pb-c.[ch] files are now included in the
distribution, they only depend on the <google/protobuf-c/protobuf-c.h>
header file, not the protobuf compiler.
.gitignore: Add src/*.pb-c.[ch]