fix hardcoded /usr/local/bin in boostfb formula
Fix mysql and libelf include dirs in hhvm formula

This is needed on a non-default HOMEBREW_PREFIX to build.
actually working include paths
Merge remote-tracking branch 'upstream/master' into brew_prefix

Conflicts:
	Formula/hhvm.rb
another HOMEBREW_PREFIX
Merge pull request #21 from oker1/brew_prefix

Fix hardcoded /usr/local/bin in boostfb formula