error: ‘Dtk::Core’ has not been declared

[PikachuHy]

/usr/include/libdtk-2.0/DCore/dtkcore_global.h:26:25: error: ‘Dtk::Core’ has not been declared
 #define DCORE_NAMESPACE Core
                         ^
/usr/include/libdtk-2.0/DCore/dtkcore_global.h:27:43: note: in expansion of macro ‘DCORE_NAMESPACE’
 #define DTK_CORE_NAMESPACE DTK_NAMESPACE::DCORE_NAMESPACE
                                           ^~~~~~~~~~~~~~~
/usr/include/libdtk-2.0/DWidget/dabstractdialog.h:27:48: note: in expansion of macro ‘DTK_CORE_NAMESPACE’
 class DAbstractDialog : public QDialog, public DTK_CORE_NAMESPACE::DObject
                                                ^~~~~~~~~~~~~~~~~~