leveldb: Add correct CFLAGS

leveldb does not respect custom's CFLAGS.
This CL fix this.

BUG=None
TEST=The CFLAGS is added.

Change-Id: I7ce1c49d757899723811740cc8713ad57f4127f9
Reviewed-on: https://chromium-review.googlesource.com/179378
Reviewed-by: Han Shen <shenhan@google.com>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
diff --git a/Makefile b/Makefile
index d337905..6d057e4 100644
--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@
 GOOGLE_PERFTOOLS_LDFLAGS=
 endif
 
-CFLAGS = -c -I. -I./include $(PORT_CFLAGS) $(PLATFORM_CFLAGS) $(OPT) $(SNAPPY_CFLAGS)
+CFLAGS += -c -I. -I./include $(PORT_CFLAGS) $(PLATFORM_CFLAGS) $(OPT) $(SNAPPY_CFLAGS)
 
 LDFLAGS += $(PLATFORM_LDFLAGS) $(SNAPPY_LDFLAGS) $(GOOGLE_PERFTOOLS_LDFLAGS)