Allow gcc-4.2 in HOMEBREW_CC

Fixes:

$ HOMEBREW_CC=gcc brew irb
...
irb(main):001:0> require 'superenv'
=> true
irb(main):002:0> ENV.setup_build_environment
=> nil
irb(main):003:0> ENV.setup_build_environment
Warning: Invalid value for HOMEBREW_CC: gcc-4.2