[QBS] Default compiler and Qt versions

[Ruslan Nigmatullin]

It's forbidden for build system (for *nix systems, like launchpad/obs) to change anything in user's home dir during building of another application (machine state before and after build process mustn't change). Currently I have to execute 'qbs platforms' and 'qbs setup-qt' to start build process. Otherway qmake/cmake/etc are able to build applications right after installation.

I guess that QBS should install something like settings for default compiler/qt versions (and they will be modified by maintainers somehow on Qt's path changed). But there is one issue: compiler should be determined during build from CC environment variable, or, if it's not set, by some other factors. How is it possible to be implemented?

Ruslan