configure.ac: check for a libprotoc header file
configure.ac: check for a libprotoc header file

it's possible for the <google/protobuf/compiler/> header files to be
shipped in a separate package (e.g., debian's libprotoc-dev). check for
this at configure time rather than allowing the build process to fail.

Sannis/protobuf-c,hongbinz/protobuf-c,pepsi7959/protobuf-c,virqin/protobuf-c,koolhazz/protobuf-c,pepsi7959/protobuf-c,tutu12777/protobuf-c,y12uc231/protobuf-c,lanniaoershi/protobuf-c,hongbinz/protobuf-c,y12uc231/protobuf-c,tutu12777/protobuf-c,koolhazz/protobuf-c,hongbinz/protobuf-c,y12uc231/protobuf-c,lanniaoershi/protobuf-c,Sannis/protobuf-c,Sannis/protobuf-c,koolhazz/protobuf-c,pepsi7959/protobuf-c,virqin/protobuf-c,lanniaoershi/protobuf-c,virqin/protobuf-c,tutu12777/protobuf-c