check for existence of homebrew-apxs

From pull request #521, we ran into a problem of having different path for homebrew apxs.

Formula httpd from homebrew-dupes installs apxs to `/usr/local/sbin/apxs` while apache24 from djl-homebrew-apache2 puts it in `/usr/local/bin/apxs`

This pull request check for the existence of apxs first to make sure that apache_apxs exist