Resurrecting clang external toolchain support in Makefile.inc

[Peter Grehan]

Hi Warner,

> I like this patch better than the one I have. I’m not 100% sure the
> ‘else’ clause will work

  The else for CROSS_BINUTILS_PREFIX ? If so, yes, that appears to work: 
I'm building with in-tree binutils.

  Ok for this to be committed ?

later,

Peter.