Provide a way to read QMAKESPEC

It would be useful to read QMAKESPEC in the project file. An example for this is shown in the documentation.

This example doesn't actually work unless you specify QMAKESPEC as a qmake variable (i.e. qmake QMAKESPEC=linux-g++ ). If you either pass '-spec linux-g++' or use the env variable QMAKESPEC, the message will be empty...