mk/compiler/gfortran.mk: Clean up version selection

  - Document hwat this file is supposed to do.  Notably, it is not
    intended to select flang.  The purpose is to find and set up for
    gfortran.

  - Only try to match the compiler version if the compiler is gcc.

  - Attempt to document the reasons behind the version choice on Darwin.

  - Try the POSSIBLE_GFORTRAN_VERSION regardless of whether gcc is in
    use - only check if it exists.

Tested on older macOS x86, NetBSD 10 amd64, NetBSD 9 earmv7hf-el to
build math/blas.

(gdt)