Coverage for Fortran compiler macros.

Coverage for Fortran compiler macros.

* tests/fortran.at (AC_OPENMP and Fortran 77)
(AC_OPENMP and Fortran): Simplify, using AT_CHECK_CONFIGURE.
(AC_F77_DUMMY_MAIN usage, AC_FC_DUMMY_MAIN usage)
(AC_F77_MAIN usage, AC_FC_MAIN usage, AC_F77_FUNC usage)
(AC_FC_FUNC usage, AC_FC_SRCEXT usage, AC_FC_FREEFORM): New
tests.
* doc/autoconf.texi (Fortran Compiler): Use GNU coding style
on C code snippets.  Add markers for tested examples.
Suggest AC_FC_FREEFORM for source file extensions which the
compiler might not natively support but which are accepted
with help from AC_FC_SRCEXT.  Suggest AC_CONFIG_HEADERS for
setups using one of the AC_*MAIN macros.

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