configure.ac: make pkg-config optional when building with --disable-protoc
configure.ac: make pkg-config optional when building with --disable-protoc

if pkg-config is installed, the libprotobuf-c .pc file will be
installed; if pkg-config is not installed, the .pc file won't be
installed.

this behavior only applies when we're building with ./configure
--disable-protoc, since pkg-config is required in order to detect the
protobuf dependency.

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