Don't care about signed/unsigned compare

Summary:
We need to stop these:
facebook/rocksdb#99
facebook/rocksdb#83

Test Plan: no

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16905

Author: igorcanadi

Makefile

@@ -36,7 +36,7 @@ else
 	PLATFORM_CCFLAGS += $(JEMALLOC_INCLUDE) -DHAVE_JEMALLOC
 endif
 
-WARNING_FLAGS = -Wall -Werror
+WARNING_FLAGS = -Wall -Werror -Wno-sign-compare
 CFLAGS += -g $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT)
 CXXFLAGS += -g $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual