scons: Whenever possible decide what to do based on platform and not compiler.

scons: Whenever possible decide what to do based on platform and not compiler.

Because compilers like GCC and Clang are effectively available everywhere
so their presence/absence is seldom conclusive.

Furthermore, all compilers we use now have stdint.h.

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