Remove GMock Doctor.
It has not been actively supported for a while.
We also now have better techniques to improve compiler errors where needed
instead of requiring a separate tool to diagnose them.
PiperOrigin-RevId: 244196068
I made a few updates to the Makefile.am files so that "make dist"
succeeds and produces a usable tarball. We need this for protobuf
because the protobuf tarballs include a bundled copy of googletest.