…sscompiling

Before this commit, the protobuf portfile when crosscompiling was not defining the
protobuf::protoc target in its CMake config files, even if its definition was instead
correctly patched in the port to point to the host version of the tool. Furthermore,
the protobuf::libprotoc library (that is a library that can also be used in
cross-compilation build) was not installed.

… options

Similar to the protobuf case, when cross-compiling it is required
that the port is already installed in the host machine triplet,
for now just x86-windows as for protobuf.

…tion-msgs-support-crosscompilation