This workaround makes the build work exactly as it did when libzrtp
was installed as a system library.  By default gcc ignores warnings
caused by system headers, so for the moment we'll continue to pretend
that libzrtp headers come from the system.

I hate this file; we need to do something about this.

Partially revert "fix libzrtp build"
commit 0b6023b2

Thanks to Ken Rice for working with me late into the night to get this
done.

Previously in this case it was only being set after the dialplan had
already executed.