maint: disable prohibit_always-defined_macros syntax check.

maint: disable prohibit_always-defined_macros syntax check.

Aside from Apple grep silently failing to run this test and
report violations, libtool still uses non-gnulib versions of
dirent.h and argz.h which use the same macros, but are not
always-defined.
* cfg.mk (local-checks-to-fix): Move
sc_prohibit_always-defined_macros from here...
(local-checks-to-skip): ...to here.
Reported by Peter O'Gorman.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>