BLD: Change Fortran version flag and string check

The version check flag for GnuFCompiler and Gnu95FCompiler were changed from
`--version` to `-dumpversion`. This simplifies the gnu_version_match code for
gfortran, and makes it possible to drop much of the check code for g77 as
well. This fix addresses issue #5315 and #5321.
I don't understand the second `-` in `([0-9-.]+)`. Is it supposed to function as `\-`? Just curious, I hadn't seen that usage before and the regular expression documentation doesn't mention it anywhere I can find.