When CMAKE_CXX_FLAGS_RELEASE is simply null, string will just receive
three params and this raises an error. Just quote this variable to
ensure it's a string.
Log: fix cmake failure when env is null
cmake -DDVERSION=6.0.0 to build libdtk6core
install qdbusxml2cpp-fix to DCore/bin to avoid conficts
do not build doc when build dtk6
remove Core5Compat depends
DVERSION ==> DTK_VERSION
tools install to CMAKE_INSTALL_LIBEXECDIR