Don't search for pkg-config.

Nothing in the rest of the build system uses it.
Don't use include() on FindProtobuf.

I have no idea what include(FindProtobuf) was doing there, especially
since it's *also* being included properly via find_package.
Merge branch 'qtquick-attempt' into winbuild

* qtquick-attempt:
  Don't use include() on FindProtobuf.
  Don't search for pkg-config.