qt5 fixed macdeployqt path to plugin dir

Using the current macdeployqt from homebrew's qt5 will result in errors like these:
```
ERROR: file copy failed from "/Developer/Applications/Qt/plugins/platforms/libqcocoa.dylib" 
```
because macdeployqt is looking for the Qt5 plugin directory in `/Developer/Applications/Qt/plugins` and not in the prefix of qt5 (e.g. `/usr/local/Cellar/qt5/5.2.0/plugins`).

This PR replaces the hardcoded plugin directory in maydeployqt with the plugin directory in prefix.
As similar patch is done by macports for qt4 (they don't have a qt5 port atm): https://trac.macports.org/browser/trunk/dports/aqua/qt4-mac/files/patch-tools_macdeployqt_macdeployqt_main.cpp.diff