add sip_require_timer=true variable to enable require timer on session refresh that breaks finicky endpoints

gsmopen: AT+CREG erratic behavior does no more give errors or warnings when badly formatted unsolicited signaling

This lets you specify a FS_SOUNDS_DIR in which to look for the sound
tarballs.  This makes it easier to use git clean -fdx without
downloading the sounds every time.

Previously if --enable-core-odbc-support was set but unixodbc-dev was
not available, we would fail during make rather during configure.