Resurrecting clang external toolchain support in Makefile.inc

[Warner Losh]

> On Feb 11, 2015, at 7:14 PM, Peter Grehan <grehan at FreeBSD.org> wrote:
> 
> 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 ?

Yes. I’m happy.

Warner