add support for the flang compiler

cmake should correctly identify the flang compiler, cf. https://github.com/flang-compiler/flang, a LLVM-based Fortran compiler.

Until recently it was identified as PGI, probably because it defined the preprocessor macro __PGI. However, with the commit https://github.com/flang-compiler/flang/commit/75c13fd83f731fd567ab4466060c7a3f1ace1ca2, the __PGI macro was removed and __FLANG was added instead, so cmake should probably detect flang based on this macro. Note that the version number is available via __FLANG_MAJOR__, __FLANG_MINOR__ and __FLANG_PATCHLEVEL__.

Cheers, Janus