- committed patch 1710671 to osx-build.sh from John Faith which allows prior detectio...

- committed patch 1710671 to osx-build.sh from John Faith which allows prior detection of the variable $INSTALLPREFIX in the environment. The other solution would be to supply it as a command line parameter, the code is here but commented at the moment. We should decide which solution is the most pratical
- additional file existence checks in osx-build.sh
- added checks for success of each step of osx-build before proceeding to the next one (via exit status)
- split the PACKAGE action into a new PACKAGE action (creates the app bundle) and a DISTRIB action (creates the dmg image)
- added an exit status for osx-app and osx-dmg
- removed the old call to osx-dmg at the end of osx-app (it has been commented for a while anyway)