Configuring Qmake mkspecs after it's built

[wysota]

1) Note the value of the qmake variable QMAKE_MKSPECS
QMAKE_MKSPECS:/usr/share/qt4/mkspecs
2) Changing the value of the $QMAKESPEC env variable to a non-existant location doesn't cause any error when running qmake.
3) I get invalid qt version message in QtCreator when using qmake and trying to set a cross compile tool chain, regardless of whether $QMAKESPEC is set. It configures ok for local build which uses the qmake's qmakespec as configured for build

Is it possible to do what I'm trying to do and I'm just missing something?

For me it works:

!build_pass {
    linux-*:message(LINUX)
    win32-*:message(WINDOWS)
}

!build_pass {
    linux-*:message(LINUX)
    win32-*:message(WINDOWS)
}

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QMAKESPEC=/home/wysota/Qt/5.3/gcc_64/mkspecs/win32-g++ qmake

yields:
Project MESSAGE: WINDOWS

Changing the specs will not modify the value of qmake variables, why would it? You are still within the same qmake installation.

Also remember you can always pass a different specs to qmake using the -spec parameter:

qmake -spec windows-g++