If QMAKESPEC is not set, then qmake will use the default. I believe this is compiled into qmake at build time. The default is whatever qt was built with.
Which brings up an important piece of advice: Always make sure the Qt libs and the QMAKESPEC match. You will run into untold linking problems if you use a win32-g++ QMAKESPEC with a Qt build for win32-msvc. If you are going to be using different compilers, you will undoubtedly need separate Qt installs. The reason is that the C++ ABI is not standardized.
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