Apple does provide minimal set of tools for developing applications. It is called XCode Command line tools (less than 1GB). Regular XCode installation requires 20GB of hard which is quite waste when you consider you are going to do everything inside QtCreator and not using XCode and iOS at all, and considering your SSD is only 128GB.

When you install XCode CommandLine Tools the path for MacOSX SDK will be in /Library/Developer/CommandLineTools/ . But QMake cannot detect MacOSX sdk in that location and does not compile the project no matter what you change in qdevice.pri 

I changed my build system from QMake to CMake and now my project compiles and works fine in the exactly same setting on the same machine.

I think it would be great if we had a variable in QMake to point to MacOSX sdk location.

p.s. If you don't have XCode installed you can install XCode Command Line tools via :

xcode-select --install

p.s. I consider this as bug or a flow, because everything required is present in my system in a way which Apple itself officially provides, and QMake should have been able to detect it.