Use `grep -q` instead of `grep --quiet` to be more portable

Makefile

@@ -224,7 +224,7 @@ endif
 
 TCC_GIT_HASH=$(shell git rev-parse > /dev/null 2>&1 && git rev-parse --short HEAD || echo no)
 ifneq ($(TCC_GIT_HASH),no)
-MODIFIED = $(shell git diff | grep --quiet +++ && echo "modified ")
+MODIFIED = $(shell git diff | grep -q +++ && echo "modified ")
 $(X)tcc.o : DEFINES +=  -DTCC_GIT_HASH="\"$(MODIFIED)$(TCC_GIT_HASH)\""
 endif