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.
Can you point me to an example of overriding the `:optional` description? I can't find a single example via ` grep ":optional" /usr/local/Library/Formula/*` and I don't see how to do it in the [Formula Cookbook](https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Formula-Cookbook.md#adding-optional-steps).