This BUILD_CONFIG option is a bit like bootstrap-debug, but rather

than comparing stripped object files, it compares compiler internal

state during stage3. Both can be used simultaneously.

This makes it slower than bootstrap-debug alone, for there's

additional dumping and recompilation during stage3.

bootstrap-debug-big can avoid the recompilation, if plenty of disk

space is available.

STAGE2_CFLAGS += -fcompare-debug= STAGE3_CFLAGS += -fcompare-debug do-compare = $(SHELL) $(srcdir)/contrib/compare-debug $$f1 $$f2