Don't use clang by default in auto-build style envs
Don't use clang by default in auto-build style envs

That is, don't use clang for Travis and sbuild. These guys play
with standard env variables at will, and using clang by default
just makes things difficult.

In particular, for Travis, using clang by default caused trouble
with ccache and the build matrix (it would set CC=gcc and then
it would get changed to clang, and badness ensued).

thatguystone/commake