Make g95 optional in math/fftw

[Martin Husemann <martin%duskware.de@localhost>]

Recently math/fftw (which is used by e.g. audio/libsamplerate) grew a g95
dependency.

Can we please split this out into a separate pkg (if it is used anywhere)
or make it an option?

The configure script has:

  --disable-fortran       don't include Fortran-callable wrappers

so implementation should be easy.

I'm asking because g95 adds like ~1 week of build time for a pkg update
now on my poor mp3 machine ;-)

Martin