fix build error: include flags in test build

util.test includes our "common.h" which in turn includes <libintl.h>
which at least on MacOS needs special compiler flags to be found.

To fix errors on the MacOS test build, we need to use the GBSCFLAGS
that configure has determined during the test build.

Makefile

@@ -592,7 +592,7 @@ config.mk: configure
 
 %.test: %.c
 	@echo TEST $<
-	$(Q)$(HOSTCC) -DENABLE_TEST=1 -o $@$(binsuffix) $< -lm
+	$(Q)$(HOSTCC) -DENABLE_TEST=1 $(GBSCFLAGS) -o $@$(binsuffix) $< -lm
 	$(Q)./$@$(binsuffix)
 	$(Q)rm ./$@$(binsuffix)