allow 'make CPPFLAGS=<something>' work again

this makes this hack a bit more bearable
for poor souls who need to pass custom
preprocessor flags to the build process

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161240 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/TargetInfo/Makefile

@@ -10,6 +10,6 @@ LEVEL = ../../../..
 LIBRARYNAME = LLVMPowerPCInfo
 
 # Hack: we need to include 'main' target directory to grab private headers
-CPPFLAGS = -I$(PROJ_OBJ_DIR)/.. -I$(PROJ_SRC_DIR)/..
+override CPPFLAGS += -I$(PROJ_OBJ_DIR)/.. -I$(PROJ_SRC_DIR)/..
 
 include $(LEVEL)/Makefile.common