Fix checks for gcc compiler when not present and --env=inherit specified

Shaun, this small patch fixes an issue with searching for homebrew gcc and
failing when --env=inherit is specified. I tried a couple of suggestions
from our earlier discussion (Homebrew/linuxbrew#29) but it still seemed
like inherit was the cleanest way to go. Happy to explore the gcc bottle
idea in the future.
Fixes chapmanb/bcbio-nextgen#605. Fixes chapmanb/bcbio-nextgen#606.
chapmanb/bcbio-nextgen#604