This is based on @haberman's changes in #8257. Now that we're using
std::mutex we no longer need to check whether pthreads are available, so
this commit removes references to HAVE_PTHREAD. I left the autotools
build alone, though, since we are likely to drop support for it soon
anyway.
In protobuf-module.cmake: in addition to checking for release or debug
variants of protoc's location we should also check the configuration "noconfig".
A series of improvements:
- Improved Protobuf module compatibility (disabled by default);
- Hide advanced settings;
- Added build tree configuration;
- Added build of examples.
