Commit "Modernize GNU compiler info on Windows" (2009-12-02) reorganized
GNU flags on Windows but let -fPIC slip through for compilation of
objects in shared libraries. While this flag is valid on most GNU
compiler platforms we need to suppress it in Windows-GNU.cmake just as
we already do in CYGWIN-GNU.cmake.
This moves GNU compiler info on Windows into new-style modules
Platform/Windows-GNU-<lang>.cmake
using language-independent helper module
Platform/Windows-GNU.cmake
to define macros consolidating the information.