Use CC in Makefiles so Travis gcc/clang variants will use gcc/clang, respectively

[johnkerl]

At present, both the gcc and clang builds at https://travis-ci.org/johnkerl/miller are actually using gcc since that's how Miller's makefiles are written.

[jungle-boogie]

I support this! It means I won't need to specify clang with each build via make -e CCOMP=clang.

👍

[johnkerl]

Right. I wrote make -e CCOMP=clang not knowing any better, but then I took a look at the Travis logs & realized how the rest of the world does it. :^/

[johnkerl]

Fixed by 610c2dc