Forbid Clang for Windows

Clang and MinGW are not expected to work together currently, and you get
strange errors when this is attempted. Instead, error out with a clear
error message.

Before the windows build rewrite, we'd just explicitly set my_clang to
false without checking what the module asked for. Now, error out if the
module explicitly asked for clang, since by default they'd have it
already if it wasn't for windows. That way when Clang+Windows does
become possible, it can gradually be tested by setting LOCAL_CLANG to
true.

Change-Id: I9e0b0dca30946d94894119443f2fd0593cee1793
diff --git a/core/binary.mk b/core/binary.mk
index dc43463..f152382 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -212,10 +212,15 @@
 # clang is enabled by default for host builds
 # enable it unless we've specifically disabled clang above
 ifdef LOCAL_IS_HOST_MODULE
-    ifneq ($($(my_prefix)OS),windows)
-    ifeq ($(my_clang),)
-        my_clang := true
-    endif
+    ifeq ($($(my_prefix)OS),windows)
+        ifeq ($(my_clang),true)
+            $(error $(LOCAL_MODULE_MAKEFILE): $(LOCAL_MODULE): Clang is not yet supported for windows binaries)
+        endif
+        my_clang := false
+    else
+        ifeq ($(my_clang),)
+            my_clang := true
+        endif
     endif
 # Add option to make clang the default for device build
 else ifeq ($(USE_CLANG_PLATFORM_BUILD),true)