Fortran checks should be optional

[biddisco]

On my windows desktop PC, the cmake configuration checks for fortran cause the spawning of
"C:/Program Files (x86)/Microsoft Visual Studio 14.0/Common7/IDE/devenv.exe" CompilerIdFortran.vfproj /build Debug
Which causes hangs for up to minutes at a time. This is probably because I have many older versions of visual studio installed and with them used the Intel compiler suite with fortran integration.

We should have an option for HPX_WITH_FORTRAN which should be OFF by default (at least on windows) and only enabled when the user wants to build fortran examples using HPX.