If both Python 2 and Python 3 are found, then build bindings for both of
them during the build process. Currently, one version of Python is
detected automatically, and building for the other requires changes the
CMake config.
The largest chunk of this change generalizes OpenCVDetectPython.cmake to
find both a Python 2 and Python 3 version of Python. Secondly, the
opencv_python module is split into two modules, opencv_python2 and
opencv_python3. Both are built from the same source. but for different
versions of Python.
Update to cmake files for to include minimum versions, and tidy up gtk operation.
Files updated:
CMakeLists.txt:
WITH_GTK now uses Gtk3 by default. If not found then Gtk2 is used.
WITH_GTK_2_X forces Gtk2.x use
cmake/OpenCVFindLibsGUI.cmake
Updated selection logic to implement methodology described above.
Implemented warning if Gtk3 not found (and not overridden)
Implemented error if Gtk does not meet minimum required version
cmake/OpenCVMinDepVersions.cmake
Added minimum Gtk version of 2.18.0
