VS: Restore CMAKE_GENERATOR_FC variable

In commit bed6c388 (VS,Xcode: Remove unused CMAKE_GENERATOR_* variables,
2013-02-19) we removed this variable among others from the VS generator
claiming they were all not used.  In fact the Fortran compiler detection
module CMakeDetermineFortranCompiler.cmake uses CMAKE_GENERATOR_FC even
for the Visual Studio IDE generators because it does not use the full
IDE compiler id detection like the C and C++ language files do.
Merge topic 'ide-compiler-vars-Fortran' into next

9f94181 VS: Restore CMAKE_GENERATOR_FC variable