Merge various launchers scripts. Slightly reorganize things.

This makes the installer launcher script to find its own location. The script is thus relocatable, which was the main problem preventing TGZ packaging... 

Not sure why we ship the script for RPM and DEB though... but no sure either how to make an installed file dependent on the type of the packaging. Well.....
This should not be set to OFF for Mac. It enables the compilation of dtkDeploy which is necessary for OSX packaging.