cairo: depend on freetype when building without X

harfbuzz won't build without cairo-ft.
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
Refactor Requirement.expand
Allow explicit conversion of requirements to deps
Revert "Remove default_formula from MPIDependency."

This reverts commit 9aaa89dbe832387249ab3ea4e22a8018cc9fe30a.