Make backup & source file blocking work with v2.3+

As of Apache v2.3, the `Order`, `Allow` & `Deny` directives are replaced
by `Require`, making the Apache v2.2.x configuration not compatible with
Apache v2.3+: http://httpd.apache.org/docs/trunk/upgrading.html#access.

This patch enables the correct syntax for both Apache v2.2.x & v2.3+, by
checking for the presence of the `mod_authz_core` base module (available
only in Apache v2.3 and later).

See also: http://httpd.apache.org/docs/2.4/mod/mod_authz_core.html

Close #5 and #6.