Use __SIZEOF_POINTER__ which the GNU Fortran compiler defines at least
on 64-bit MinGW. Assume default size 4 on MinGW if gfortran does not
define the size.
Implement Fortran 32/64-bit ABI detection on some platforms. We need to
set CMAKE_SIZEOF_VOID_P correctly in Fortran-only projects so that the
find_library() command knows whether to look for 64-bit binaries. We
also detect ELF binaries to enable RPATH replacement. See issue #10119.