This BUILD_CONFIG option tests that target libraries built during

stage3 would have generated the same executable code if they were

compiled with -g0.

It uses -g0 rather than -gtoggle because -g is default on target

library builds, and toggling it where it's supposed to be disabled

breaks e.g. crtstuff on ppc.

STAGE1_TFLAGS += -g0 -fcompare-debug= STAGE2_TFLAGS += -fcompare-debug= STAGE3_TFLAGS += -fcompare-debug=-g0 do-compare = $(SHELL) $(srcdir)/contrib/compare-debug $$f1 $$f2