cantera: new formula for Cantera 2.1

The build system for Cantera was replaced in Cantera 2.0 (now using SCons instead of make), so this is basically an entirely new formula.

It should work with any compiler, but its necessary to be explicit about the compilers to avoid ending up with mismatched versions of the C++ standard library between the parts that are compiled by SCons (which will default to using 'clang++' and use libc++) and Python's distutils (which will call 'g++' and use libstdc++, nevermind that it's just an alias to Clang).

Is there a way to query which compiler should be used?