scons: Support Clang on Windows.

scons: Support Clang on Windows.

- Introduce 'gcc_compat' env flag, for all compilers that define __GNUC__,
  (which includes Clang when it's not emulating MSVC.)

- Clang doesn't support whole program optimization

- Disable enumerator value warnings (not sure why Clang warns about them,
  as my understanding is that MSVC promotes enums to unsigned ints
  automatically.)

This is not enough to build with Clang + AddressSanitizer though.  More
follow up changes will be required for that.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>