Build fails on Ubuntu 12.04 due to probed Qt dirs not used

Patch ea45279 added an assumption that the Qt headers are installed in include/ subdirectory of a project root. However, if Qt4 is provided by system packages on Linux, the headers and libraries will no longer be
sibling directories (i.e. it is /usr/include/qt4 instead of /usr/local/qt4/include).

This patch restores the old functionality of using the probed variables. (I haven't tested whether these really should be used for Windows also; I think that should be considered).