www/mod_pagespeed: Fix undefined symbol error while loading module in apache24
tl;dr, The error is caused by the build-system in "apache24" case can not
detect that FreeBSD supports "posix-sharedmem" in the pthread/libthr.

Reported by: Marcarian Alexandru <marcarianalexandru@gmail.com>
Sponsored by: Netzkommune GmbH