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 in Qt5 plugin directory in /Developer/Applications/Qt and not in the prefix of qt5.

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