'mac': it applies both on Mac OS X and iOS
'macx': it is specific to Mac OS X.
So, if you wanna include iOS support later, or simply be flexible (and why not?), you will better use 'mac'. Otherwise go for the latter. So you will need either of those.
Here you can find the official documentation:
http://qt-project.org/doc/qt-5.1/qmake/qmake-language.html#platform-scope-values
Here are the variants you may need based on your specific use case.
mac (including iOS support)
win32 {
SOURCES += hellolinux.cpp
} mac {
SOURCES += hellomac.cpp
}
macx
win32 {
SOURCES += hellolinux.cpp
} macx {
SOURCES += hellomacx.cpp
}
Here you can find the source code to seek more information about undocumented scopes:
http://qt.gitorious.org/qt/qtbase/source/730bc064a070e886e10950ccfd59780e8976f5fd:mkspecs