fix clang warning

mk/vars.mk

@@ -118,7 +118,7 @@ endif
 
 # A nifty macro to make testing gcc features easier (from uClibc project)
 check_gcc=$(shell \
-        if $(CC_FOR_BUILD) $(1) -S -o /dev/null -xc /dev/null > /dev/null 2>&1; \
+        if $(CC_FOR_BUILD) $(1) -Werror -S -o /dev/null -xc /dev/null > /dev/null 2>&1; \
         then echo "$(1)"; else echo "$(2)"; fi)
 
 CF_FOR_BUILD=$(call check_gcc,-fhonour-copts,)