Use $ORIGIN to load libs from program directory

The RPATH attribute won't expand relative directories (such as ".")
neither can it be a semi-colon separated list; the old way of setting
it was wrong and would lead to libraries being loaded from system
directories instead.
Refactor variable containing filenames for license

These will be used in several scenarios later.
Optionally enable system installation of binaries

If the option PRIVATE_INSTALL is set to OFF, LSB-compliant paths are
used for the `install' target.
Path to Octave plugins is taken from conf. program
Put launcher for ResInsight in desktop environment
Merge pull request #15 from rolk/instdir

Enable installation of ResInsight with a package manager through an option