Add support for the gsm lossy speech compression codec

Packages describing “gsm” as local USE flag

Package “gsm” Flag Description
net-misc/asterisk-moh-opsound Install the sounds files for the gsm codec.
net-misc/asterisk-extra-sounds Install the sounds files for the gsm codec.
net-voip/yate use GSM codec

All packages providing a “gsm” USE flag (6)