This fixes an issue with inconsistent startup scripts.
LD_LIBRARY_PATH was incomplete since at least 5.0 (5.12 is broken, too)
likewise ctrl surface path were outdated, etc.
Prefer to use `gtk2_ardour/ardev_common.sh.in` as the central
point to define environment variables for running Ardour from
the source-tree.
Other start scripts e.g. `vst/ardevst` already did this.