configure.ac: disable building libgsmhr by default

At the moment, Half Rate codec support is limited. There are still
some issues with encoding of the reference files (see OS#2514),
and moreover AddressSanitizer is not happy about libgsmhr...

Change-Id: Ib86f3f972fc31c7eedf7ed75a7c356ac62e47dc6
Related: OS#2514
1 file changed