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