GCC: unset env vars that cause build errors

Unset some environment variables that can cause nasty build errors when
bootstrapping GCC.
adjust use of unwanted_env_vars to easyconfig parameter
Merge pull request #1 from boegel/gcc-unset-env

adjust use of unwanted_env_vars to easyconfig parameter
use self.cfg.update for adjusting unwanted_env_vars
Merge pull request #2 from boegel/gcc-unset-env

use self.cfg.update for adjusting unwanted_env_vars
Merge pull request #237 from wpoely86/gcc-unset-env

GCC: unset env vars