libiberty, configure, Darwin: Avoid detecting deprecated sbrk.

libiberty, configure, Darwin: Avoid detecting deprecated sbrk.

Darwin provides an implementation of sbrk, which is detected by
the configuration process.  However, it is deprecated which leads
to build warnings.  The malloc-based implementation is more
suitable.  This patch removes sbrk from the functions searched
for Darwin.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
libiberty/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Do not search for sbrk on Darwin.
	* xmalloc.c: Do not declare sbrk unless it has been found
	by configure.