Fix default Makefile with clang on Travis.

If you check the merge for #206 that's [built on Travis](https://travis-ci.org/hcatlin/libsass/jobs/15677895), you'll see that it doesn't actually use `clang++` with the default makefile (`AUTOTOOLS=no` and `CC=clang`).

That's because the `Makefile` unconditionally sets the `CXX` variable which is higher priority than the environment variable that Travis uses.