* c-opts.c (c_common_post_options): PCH is not compatible with
	no-unit-at-a-time.
	* opts.c (handle_options): Enable unit-at-a-time at O0 along with
	-fno-toplevel-reorder by default now.
	* gcc.dg/weak/weak-2.c: We no longer complain about incompatibilty.
	* gcc.dg/weak/weak-3.c: We no longer complain about incompatibilty.
	* gcc.dg/weak/weak-4.c: We no longer complain about incompatibilty.
	* gcc.dg/weak/weak-5.c: We no longer complain about incompatibilty.
	* gcc.dg/weak/weak-6.c: Fix thinko in previous change.
	* gcc.dg/weak/weak-7.c: Likewise.

From-SVN: r136903