abstract-php: Enable building embed target

Adds an option to build the PHP embed target (PHP as a library).  This
feature can be used with uWSGI, for instance.
Uh !! Your update has revealed me to a configuration error I have never noticed before, for `fpm`. I have to fix this.

Anyway, as `with-embed` implies ` without_apache`, you should have `!build.with?('embed'))` instead