res_pjsip,compat: INFINITY and NAN undefined

On some systems the values for INFINITY and NAN are not defined thus causing
a build error on those systems.  Added definitions for those if they had
not previously been defined.

(closes issue ASTERISK-23056)
Reported by: capouch
Patches:
     inf-nan-patch.txt uploaded by capouch (license 6564)
........

Merged revisions 406788 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406789 65c4cc65-6c06-0410-ace0-fbb531ad65f3