issues with configuring the trunk to build Fortran

[Joe Buck]

Attempting to configure the trunk on an i686 gnu/linux box, with
--enable-languages=c,c++,objc,f95

fails.  I get

...
checking for correct version of gmp.h... yes
checking for MPFR... no
The following requested languages were not found: f95

even though there is a gcc/fortran/config-lang.in file with the line

language="f95"

Doing some googling, it seems that MPFR is now required; if that is the
issue, we should have a better failure message.  f95 is found; if the
prerequisites are not present we should say so.