Allows suppression of .pb-c.{c,h} file generation on a .proto file
basis.

protobuf-c.proto: Use the new option in itself, because the options are
never used by protobuf-c runtime, only by the protoc-gen-c compiler.

t/test-full.proto: import protobuf-c.proto to test that
protobuf-c.pb-c.h dependency does not get included in test-full.pb-c.h.