Make it an error when LOCAL_CLANG is false.

Only projects in LOCAL_CLANG_EXCEPTION_PROJECTS can set LOCAL_CLANG to false.
Add external/gentoo/integration to this white list.

Bug: 30575506
Change-Id: Icd508686bc2eb8b9d400200166df80115af20da1
Test: build and find new warning in build.log
diff --git a/core/binary.mk b/core/binary.mk
index 9181699..e225edb 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -278,7 +278,7 @@
 # in the exception project list.
 ifeq ($(my_clang),false)
     ifeq ($(call find_in_local_clang_exception_projects,$(LOCAL_MODULE_MAKEFILE)),)
-        $(warning $(LOCAL_MODULE_MAKEFILE): $(LOCAL_MODULE): warning: LOCAL_CLANG is set to false)
+        $(error $(LOCAL_MODULE_MAKEFILE): $(LOCAL_MODULE): LOCAL_CLANG is set to false)
     endif
 endif