install_prereq: Tweak flags when configuring pjproject.

This change does two things:
1. Disables debugging so assertions which can return an error do,
instead of asserting.
2. Enables IPv6 support.

ASTERISK-24632 #close
Reported by: Rusty Newton
........

Merged revisions 431843 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431844 65c4cc65-6c06-0410-ace0-fbb531ad65f3