Request for review: build infrastructure for Clang

[M. Warner Losh]

In message: <4BF407EF.7010508 at FreeBSD.org>
            Doug Barton <dougb at freebsd.org> writes:
: ... which leads to hopefully my last suggestion, if there is _any_
: chance that it would be useful to have tblgen installed then providing
: a knob to do so (defaulting to off of course) is cheap, and will
: prevent you having to rework the code later when someone requests it
: (and believe me, someone will). :)

Knobs are too cheap.  We're starting to accumulate rather a lot of
them for trivial things...  We need to think about a better way to do
subsetting...

Warner