compilerplugins: -isystem /usr/include breaks the build

... on Fedora 24, and it's a Bad Idea in general because it breaks
"#include_next" ordering of GCC's and glibc's variously distributed
headers.

Change-Id: I0c8b51a6d7191ec9cc5187636ea2717e8fd97715

compilerplugins/Makefile-clang.mk

@@ -25,7 +25,8 @@ CLANGCXXFLAGS=-O2 -Wall -Wextra -Wundef -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=-isystem $(CLANGDIR)/include -isystem $(CLANGDIR)/tools/clang/include
+CLANGINCLUDES=$(if $(filter /usr,$(CLANGDIR)),,-isystem $(CLANGDIR)/include) \
+			  -isystem $(CLANGDIR)/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