MinGW (Windows) builds do not support Clang

Use the default for host builds: linux and darwin use clang, windows
will use gcc.

This is the same behavior as before I rewrote the windows support, where
we were explicitly disabling clang in the build system for windows
builds. I'm making the build error out when setting LOCAL_CLANG := true
on windows modules instead of overriding the selection in the build
system so that it will be easier to gradually test clang for windows in
the future.

Change-Id: I7b293923cf5fc8f066c848179e6f025136d105a3
1 file changed