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likewise ctrl surface path were outdated, etc.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.
uselib is no longer implicit (inherited by .use). This is still incomplete,
some uselibs for non-linux variants may be missing.
bld.is_defined("HAVE_XXX") also no longer works and will have to be
changed (I think to bld.env["HAVE_XXX"]) in countless places.
