Identifier for win64 configuration in Qmake
Since Qt5 you can use QT_ARCH to detect whether your configuration is 32 or 64. When the target is 32-bit, that returns i386 and in case of a 64-bit target it has the value of x86_64. So it can be used like:
contains(QT_ARCH, i386) { message("32-bit")} else { message("64-bit")}