fortran: use grep instead of fgrep

fortran: use grep instead of fgrep

fgrep has been deprecated in favor of grep -F for a long time, and the
curren grep release (3.10) prints a warning of fgrep is used.  Stop
using fgrep so we won't see the warning.

We can't hard code grep -F here or it may break build on hosts w/o GNU
grep.  autoconf documentation contains a warning about this issue and
suggest to use AC_PROG_FGREP and $FGREP, but these are too overkill in
the specific case: there is no way "debian" could be interpreted as an
non-trivial regex, so we can use a plain grep here.

gcc/fortran/ChangeLog:

	* Make-lang.in: Use grep instead of fgrep.