scons: Move fallback HAVE_* definitions to headers.

scons: Move fallback HAVE_* definitions to headers.

These were being defined in SCons, but it's not practical:

- we actually need to include Gallium headers from external source trees, with
completely disjoint build infrastructure, and it's unsustainable to
replicate the HAVE_xxx checks or even hard-coded defines across
everywhere.

- checking compiler version via command line doesn't really work due to
  Clang essentially being like a cameleon which can fake either GCC or
  MSVC

There's no change for autoconf.

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