tomcat-native: add --with-homebrew-apr option

Yosemite ships with APR 1.4.8. This new option
allows using newer versions (eg. 1.5.1) from the
homebrew/apache tap.
I would prefer we use `:optional` even if we have to lie a little bit. Declaring dependencies with ruby conditionals is really problematic so I'd like to avoid adding more if at all possible. Eventually there will be support for option names that do not correspond directly to the formula name.