As I see in debian looks like this should be compatible with 2.4 as-is :/
Since this is ~arch and unmaintained, I'm for changing "need_apache2_2" to "need_apache2" in the ebuild and seeing what happens.
+ 18 Dec 2014; Pacho Ramos <pacho@gentoo.org> mod_authn_sasl-1.2.ebuild: + Should work with 2.4 too as explained at bug #532804 +