Makefile: Pass USE_LIBPCRE down in GIT-BUILD-OPTIONS

Otherwise we would fail to rebuild correctly when this option was
changed between $(MAKE) invocations, and more importantly, $(MAKE) test
would not pass it down and t/test-lib.sh would not set the LIBPCRE
prerequisite.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2109,6 +2109,7 @@ GIT-BUILD-OPTIONS: FORCE
 	@echo PYTHON_PATH=\''$(subst ','\'',$(PYTHON_PATH_SQ))'\' >>$@
 	@echo TAR=\''$(subst ','\'',$(subst ','\'',$(TAR)))'\' >>$@
 	@echo NO_CURL=\''$(subst ','\'',$(subst ','\'',$(NO_CURL)))'\' >>$@
+	@echo USE_LIBPCRE=\''$(subst ','\'',$(subst ','\'',$(USE_LIBPCRE)))'\' >>$@
 	@echo NO_PERL=\''$(subst ','\'',$(subst ','\'',$(NO_PERL)))'\' >>$@
 	@echo NO_PYTHON=\''$(subst ','\'',$(subst ','\'',$(NO_PYTHON)))'\' >>$@
 ifdef GIT_TEST_CMP