Hi !

I've been forced to work with a lot of different Qt4 versions and decided to allow Boost.Build to configure several Qt versions instead of only one. Please find the patch attached for review. I'd like to wait for the 1.38 release in order to prevent this from being accidentally merged...

New features:
- Auto detection of Qt version using qmake (this is the way CMake uses, too).
- Specification of target-requirements in order to configure the same Qt version in different variants like debug, target-os=windows, or the like.
- new feature "qt4" which allows selection of the qt version to use (qt4=4.5.0, qt4=4.4.3). Default is the first Qt version seen by the toolset.

- Shall I add a user-specified version to the toolset initialisation ? This is the way python.jam goes and I doubt everyone will use different Qt version (and in this case, one will have one 4.5 while testing 4.5.1 :-))
- using a 64bit version of qt on windows using "<toolset>msvc <address-model>64" as toolset requirements seems not to work. Looks like the logic inspired by python.jam does not like features without default values. Any ideas welcome.
- should the feature simply be "qt" thus shortening "qt=4.5.0" ? Or should I go for "qt-version" requesting "qt-version=4.5.0" ? I'm quite at loss here.

Comments welcome !


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