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.