Ignore warnings in Clang headers

(as occur at least in trunk Clang builds)

Change-Id: I7e74200ae1042133fa04477e39e46fd2f01cc1b0

compilerplugins/Makefile-clang.mk

@@ -18,7 +18,7 @@ CLANGCXXFLAGS=-O2 -Wall -Wextra -g
 # Clang headers require these.
 CLANGDEFS=-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -fno-rtti
 # All include locations needed.
-CLANGINCLUDES=-I$(CLANGDIR)/include -I$(CLANGDIR)/tools/clang/include -I$(CLANGBUILD)/include -I$(CLANGBUILD)/tools/clang/include
+CLANGINCLUDES=-isystem $(CLANGDIR)/include -isystem $(CLANGDIR)/tools/clang/include -isystem $(CLANGBUILD)/include -isystem $(CLANGBUILD)/tools/clang/include
 
 # Clang/LLVM libraries are intentionally not linked in, they are usually built as static libraries, which means the resulting
 # plugin would be big (even though the clang binary already includes it all) and it'd be necessary to explicitly specify