Linuxbrew: Fix --env=inherit

Fix checks for gcc compiler when not present and --env=inherit specified.
Fixes Homebrew/linuxbrew#136
See chapmanb/bcbio-nextgen#604
See chapmanb/bcbio-nextgen#605
See chapmanb/bcbio-nextgen#606
cmake: Avoid python dependency for Linuxbrew

On Linux systems, with Linuxbrew, the current cmake build recipe will pull in
Python and a large number of other system packages that Python depend on. This
can cause some bad library dependency issues. This small fix skips the Python
install on these systems. Moved from Homebrew main recipe by request.
See Homebrew/homebrew#32601