Formulae can now pass build options to dependencies. The following
syntax is supported:
depends_on 'foo' => 'with-bar'
depends_on 'foo' => ['with-bar', 'with-baz']
If a dependency is already installed but lacks the required build
options, an exception is raised. Eventually we may be able to just stash
the existing keg and reinstall it with the combined set of used_options
and passed options, but enabling that is left for another day.
Optional deps are not installed by default but generate a corresponding
"with-foo" option for the formula. Recommended deps _are_ installed by
default, and generate a corresponding "without-foo" option.
