New macro AC_FC_PP_SRCEXT for preprocessed file extensions.

New macro AC_FC_PP_SRCEXT for preprocessed file extensions.

* lib/autoconf/fortran.m4 (AC_FC_PP_SRCEXT): New macro.
* lib/autom4te.in (Automake-preselections): Preselect it.
* doc/autoconf.texi (Fortran Compiler): Document it, rewriting
the documentation for AC_FC_SRCEXT along the way.
* tests/fortran.at (AC_FC_PP_SRCEXT usage): New test.
* tests/mktests.sh: Exclude the macro from default testing.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>