build: check for -lgsm

If libosmo-legacy-mgcp is built with --enable-mgcp-transcoding, we need to link
-lgsm here as well. This autodetects whether -lgsm is necessary.

Todo: how about --with-g729?

Todo: osmo-msc is only using the mgcp client and should not actually need
      transcoding nor -lgsm.

Change-Id: Iab55a089ae36017b79e7cbc3cac45ef9fd85dd43
1 file changed