New environment option, `--env=inherit`

Pass the current CC/CXX environment through to the build environment
with `--env=inherit`. This option allows passing the user's C compiler
environment variables through to the build environment and avoids
defaulting to the system gcc/g++, which does not work on module-based
Linux cluster distributions.

Closes Homebrew/linuxbrew#13
Closes Homebrew/linuxbrew#29