Allow building with Homebrew's apxs

I've made a start fixing the issue described in #401 and #437. The issue is that the path to the ````apxs```` utility was hardcoded to the one shipped with OS X. I've adjusted this so that the ````apxs```` contained within ````HOMEBREW_PREFIX```` can be used instead; you must supply the ````--homebrew-apxs```` switch to the install command for this to be the case.

I'm not sure I like this approach, though, as it'd probably be cleaner (and more consistent with other Homebrew formulae) if we merely accepted the ````apxs```` on the user's ````PATH````? I'd really appreciate your input!