Avoid reference to $CYGWIN in Fortran macros.

Avoid reference to $CYGWIN in Fortran macros.

* lib/autoconf/fortran.m4 (_AC_FC_LIBRARY_LDFLAGS): Require
AC_CANONICAL_HOST.  Replace test for $CYGWIN with $host_s test.
* tests/fortran.at (AC_F77_DUMMY_MAIN usage)
(AC_FC_DUMMY_MAIN usage, AC_F77_MAIN usage, AC_FC_MAIN usage):
Use AT_CONFIGURE_AC and simplify accordingly, so auxiliary
scripts are copied into the test directories.
(AC_F77_FUNC usage, AC_FC_FUNC usage): Likewise.  Adjust to
autoheader being used now.

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