Avoid resetting CC and GCC on Linuxbrew

Coupled with `--env=std` this allows pass through of a user's C compiler environment on Linuxbrew with environmental variables. Avoids defaulting to the system gcc/g++ which does not work on many module-based Linux cluster distributions: Homebrew/linuxbrew#13

This was the minimal patch I could generate to enable this behaviour on shared cluster systems where Linuxbrew stubbornly insisted on using a broken /usr/bin/gcc. If you set CC/GCC correctly and avoid superenv with `--env=std`, the specified compilers will get picked up and used. This fix issues building cmake and boost for me on tricky systems.

Happy to receive suggestions. Thanks much.