libgfortran: Use the libtool macro to determine libm availability.

We recently had a report of build failure against a Darwin branch on
the latest OS release.  This was because (temporarily) the symlink
from libm.dylib => libSystem.dylib had been removed/omitted.

libm is not needed on Darwin, and should not be added unconditionally
even if that is (mostly) harmless since it is a symlink to libc.

There could be cases where the addition was not completely harmless
because the presentation of the symlink would cause the symbols exposed
in libSystem to be considered ahead of ones presented in convenience
libraries.

libgfortran/ChangeLog:

	* Makefile.am: Use configured libm availability.
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use libtool macro to find libm availability.
	* libgfortran.spec.in: Use configured libm availability.