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.
@mikemcquaid If I do it this way then brew options looks like this:

    $ brew options tomcat-native
    --with-apr
    	Build with apr support
    ...

Not exactly truthful. Either way, we're building with APR support. It's either through OS X or homebrew.