Config.linux: Better check for gcc.

To print in make:
$(info IS_GCC is $(IS_GCC))

vm/Config.linux

@@ -6,8 +6,8 @@ LIBS = -ldl -lm -lrt -lpthread -Wl,--export-dynamic
 # clang spams warnings if we use -Wl,--no-as-needed with -c
 # -Wl,--no-as-needed is a gcc optimization, not required
 # we want to work with g++ aliased as c++ here, too
-IS_GCC = $(shell $(CXX) --version | grep '(GCC)')
+IS_GCC = $(shell $(CXX) --version | grep -o '(GCC)')
 
-ifdef ($(IS_GCC))
+ifeq ($(IS_GCC), (GCC))
 	SITE_CFLAGS += -Wl,--no-as-needed
 endif