Darwin, configure: Handle a missing substitution.

Darwin, configure: Handle a missing substitution.

The configure substitution for enable_darwin_at_rpath has been
omitted, which leads to a failure to set ENABLE_DARWIN_AT_RPATH in
the testsuite site.exp (which leads to failure to add -B options
in some cases, breaking uninstalled testing there).

Since we already have substitutions for ENABLE_DARWIN_AT_RPATH_TRUE
we can use that instead, which is what this patch does.

gcc/ChangeLog:

	* Makefile.in: Emit ENABLE_DARWIN_AT_RPATH into site.exp
	when ENABLE_DARWIN_AT_RPATH_TRUE is not '#'.

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