fix: compile error with gcc 9.3 which is default in ubuntu 20.04

Makefile

@@ -2,6 +2,7 @@ CLEAN_FILES = # deliberately empty, so we can append below.
 CXX=g++
 PLATFORM_LDFLAGS= -lpthread -lrt
 PLATFORM_CXXFLAGS= -std=c++11 -fno-builtin-memcmp -msse -msse4.2 
+ROCKSDB_CXXFLAGS= -Wno-error=deprecated-copy -Wno-error=pessimizing-move -Wno-error=class-memaccess -Wno-error=array-bounds
 PROFILING_FLAGS=-pg
 OPT=
 LDFLAGS += -Wl,-rpath=$(RPATH)
@@ -226,7 +227,7 @@ $(PINK):
 	$(AM_V_at)make -C $(PINK_PATH)/pink/ DEBUG_LEVEL=$(DEBUG_LEVEL) NO_PB=0 SLASH_PATH=$(SLASH_PATH)
 
 $(ROCKSDB):
-	$(AM_V_at)make -j $(PROCESSOR_NUMS) -C $(ROCKSDB_PATH)/ static_lib DISABLE_JEMALLOC=1 DEBUG_LEVEL=$(DEBUG_LEVEL)
+	$(AM_V_at) CXXFLAGS='$(ROCKSDB_CXXFLAGS)' make -j $(PROCESSOR_NUMS) -C $(ROCKSDB_PATH)/ static_lib DISABLE_JEMALLOC=1 DEBUG_LEVEL=$(DEBUG_LEVEL) 
 
 $(BLACKWIDOW):
 	$(AM_V_at)make -C $(BLACKWIDOW_PATH) ROCKSDB_PATH=$(ROCKSDB_PATH) SLASH_PATH=$(SLASH_PATH) DEBUG_LEVEL=$(DEBUG_LEVEL)