Qt4 build properties checking

[anonymous]

Need to account for the fact that Qt4 requires multithreading. See: ​http://thread.gmane.org/gmane.comp.lib.boost.build/12369/

We also need to check that on Windows, GUI sybsystem is used, quoting malcolm_cifuentes@…:

by default using qt4 and mingw seems to build a console application. This causes all qt4 gui's to open a dos console in the background. This is unexpected bahaviour and goes against qmake behaviour. In order to fix this, the linker needs to be told to use the windows subsystem (eg. <linkflags>-Wl,-subsystem,windows). This will remove the pesky console window. It would be really nice if the qt4.jam did this automatically or at the very least provided a clue some where as to how to remove the console window.

[ghost]

Not so critical. Also, depends on #118.