Since xdgiconloader is not adapted to Qt6, libxdg is disabled by default when compiling Dtk6
Do not install dconfig file in dtkgui to avoid conficts with dtk6gui
C++ 17 by default
DVERSION ==> DTK_VERSION
cmake_minimum_required 3.13
tools install to CMAKE_INSTALL_LIBEXECDIR
I change the source code from qmake to cmake. Move the files
which will be install to the /usr/include to $root/include, which will
help to mattern match and easylier to do install. and the change will
break the build of qmake
Log: from qmake to cmake
Influence: qmake is breaked