cc/gcc: fortran requires GMP and MPFR

Even if gcc itself does not require GMP or MPFR (eg. gcc-4.2 and before
don't), building the fortran frontend always required those companion
libraries.

Select them if the Fortran language is selected.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.