Introduce DISABLE_WARNING_AS_ERROR in Makefile

Summary: See https://www.facebook.com/groups/rocksdb.dev/permalink/718536351578215/. We should enable people to run without warning-as-error, since we break this so often for different compilers.

Test Plan: introduce an warning, compile sucessfully

Reviewers: yhchiang, meyering, igor.sugak, rven, sdong

Reviewed By: sdong

Subscribers: adamretter, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D33963

Makefile

@@ -124,9 +124,13 @@ install:
 	[ ! -e $(SHARED) ] || install -C -m 644 $(SHARED) $(INSTALL_PATH)/lib
 #-------------------------------------------------
 
-WARNING_FLAGS = -W -Wextra -Wall -Werror -Wsign-compare -Wshadow \
+WARNING_FLAGS = -W -Wextra -Wall -Wsign-compare -Wshadow \
   -Wno-unused-parameter
 
+ifndef DISABLE_WARNING_AS_ERROR
+	WARNING_FLAGS += -Werror
+endif
+
 CFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CCFLAGS) $(OPT)
 CXXFLAGS += $(WARNING_FLAGS) -I. -I./include $(PLATFORM_CXXFLAGS) $(OPT) -Woverloaded-virtual -Wnon-virtual-dtor