Also fix problem with incorrect symbolic link to plugins

hdiutil called from inside macdelpoyqt was failing. Managed to get it
not to fail by changing layout to SPUD instead of GPTSPUD.
Unfortunately know idea why this works!

Handles case where multiple versions of QT are installed on machine.

Files were not being copied to correct location

Files were not being copied to correct location

Settings in user_config.pri are overridden by those specified at the command line when calling qmake.
Also updated the README and user_config.pri.dist to be a little more clear about specifying these other options.