Revert "Replace deprecated OUTPUT_PATH variables with OUTPUT_DIRECTORY equivalents"

This reverts commit b74a4ecb4ab94db583c133982d29842d0a1a1cbf.

Fixes #226
fix flag used to build yarpbuilder-gtk, relevant to #231
[doc] be clearer about return value of ResourceFinder::configure, fixes #230
make Vector::data and Matrix::data return null when empty, fixes #228, fixes #229
Try to fix Qt gui build on windows
fix for windows compilation of Qt gui's
guards for windows reinstated
Properly fix linking QtMain on windows

CMake policy CMP0020 (that since CMake 2.8.11 automatically links
QtMain target when set to NEW for WIN32 targets) was discarded because
set in the YarpUseQt5 file.

The "cmake_policy" call is now in the main CMakeLists, so that it is
automatically used in all the project.
Fix CMake warnings caused by qt5_use_modules macro

The macro is now hidden on CMake 2.8.11 or later, by the one defined in
YarpUseQt5.cmake, that just calls target_link_libraries instead.