updates for ilastik compilation

Summary of the changes:
* Introduced cmake variables ILASTIK_VERSION and VIGRA_VERSION to specify the desired versions. Policy: if these variables contain the string 'master', the build will track upstream master, otherwise it compiles the specified version (which may be any git committish).
* Create startup scripts via 'configure_file()'.
* Changed startup scripts to use relative paths w.r.t. the script's location.