gfortran updates

* Enable graphite loop optimization by adding cloog and isl as dependencies.
* Bootstrapped by Clang should work now.
* Stop installing libstdc++ pretty printer, which conflicts with gcc48 formula.
* Move '--enable-language=gfortran' into args for consistency.
* Fix comments: man7 is not installed by any formula, as it is hard to
  decide which formula should install it now that there are multiple
  versioned gcc formulae.
* Use double quote for interpolation. Thanks @jacknagel.

Closes #19382.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>