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

llvm-svn: 161240
diff --git a/llvm/lib/Target/PowerPC/TargetInfo/Makefile b/llvm/lib/Target/PowerPC/TargetInfo/Makefile
index a101aa4..2d0560d 100644
--- a/llvm/lib/Target/PowerPC/TargetInfo/Makefile
+++ b/llvm/lib/Target/PowerPC/TargetInfo/Makefile
@@ -10,6 +10,6 @@
 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