There is a type mismatch on line 103 of main.cpp in the 5.1 source code:
app.setLibraryPaths(app.applicationDirPath() + "/../PlugIns");
which provides a QString as a parameter when a QStringList is required. This error is corrected by the following revision:
app.setLibraryPaths(QStringList(app.applicationDirPath() + "/../PlugIns"));
which enables compilation of the source code on Mac OS X 10.8.