pjproject_bundled: Tweaks to support out-of-tree development

pjproject_bundled:  Tweaks to support out-of-tree development

* pjproject is now configured with --disable-libsrtp so it will
  build correctly when doing "out-of-tree" development.  Asterisk
  doesn't use pjproject for handling media so pjproject doesn't
  need libsrtp itself.

* The pjsua app (which we used to use for the testsuite) no longer
  builds in pjproject's master branch so we just skip it.  The
  testsuite no longer needs it anyway.

See third-party/pjproject/README-hacking.md for more info on building
pjproject "out-of-tree".

(cherry picked from commit f9e8c3488b60803c919200e0f6edd10b7427da36)