fixlet: flags for Fortran77 compiler are in FFLAGS, not F77FLAGS

fixlet: flags for Fortran77 compiler are in FFLAGS, not F77FLAGS

This change fixes automake bug#10555.

Note that the bug was a minor one, since it didn't affect the
compilation rules generated by automake, but only only the "hints"
printed by automake in some error messages (e.g., "The usual way
to define `FFLAGS' is to add AC_PROG_F77 to configure.ac").

* lib/Automake/Variable.pm (%_ac_macro_for_var): The code generated
by AC_PROG_F77 uses FFLAGS, not F77FLAGS, as the variable where to
look for switches for the Fortran 77 compiler: adjust accordingly.