Don't use versionless Qt cmake imports (Issue #57 and #60)

I think you can only use them with Qt 5.15 and Kubuntu 20.04 only has
Qt 5.12.

CMakeLists.txt

@@ -16,8 +16,12 @@ include (KDEInstallDirs)
 include (KDECMakeSettings)
 include (KDECompilerSettings NO_POLICY_SCOPE)
 
-set(QT_MIN_VERSION "5.9.0")
-find_package(Qt5 ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS
+# set(QT_MIN_VERSION "5.9.0")
+# https://doc.qt.io/qt-6/cmake-qt5-and-qt6-compatibility.html#supporting-older-qt-5-versions
+# find_package(QT NAMES Qt6 Qt5)
+find_package(QT NAMES Qt5)
+# find_package(QT NAMES Qt6)
+find_package(Qt${QT_VERSION_MAJOR} CONFIG REQUIRED COMPONENTS
     Widgets
     DBus
 )

src/CMakeLists.txt

@@ -1,6 +1,6 @@
 find_package (KDecoration2 REQUIRED)
 
-find_package (Qt5 REQUIRED COMPONENTS
+find_package (Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS
     Core
     Gui
 )
@@ -30,7 +30,16 @@ find_package(XCB MODULE REQUIRED COMPONENTS
 )
 set_package_properties(XCB PROPERTIES TYPE REQUIRED)
 
-find_package(Qt5 ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS X11Extras)
+if (Qt6_FOUND)
+    # The QX11Info class has been removed.
+    # Clients that still rely on the functionality can include the private header <QtGui/private/qtx11extras_p.h> as a stopgap solution.
+    # To enable private headers use QT += gui-private with qmake, or add a project dependency to Qt::GuiPrivate with CMake.
+    # https://doc.qt.io/qt-6/extras-changes-qt6.html#changes-to-qt-x11-extras
+    find_package(Qt${QT_VERSION_MAJOR} ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS GuiPrivate)
+elseif(Qt5_FOUND)
+    find_package(Qt${QT_VERSION_MAJOR} ${QT_MIN_VERSION} CONFIG REQUIRED COMPONENTS X11Extras)
+endif()
+
 
 if(X11_FOUND AND XCB_XCB_FOUND)
     set(HAVE_X11 ON)
@@ -81,9 +90,9 @@ add_library (materialdecoration MODULE
 target_link_libraries (materialdecoration
     PUBLIC
         dbusmenuqt
-        Qt::Core
-        Qt::Gui
-        Qt::X11Extras
+        Qt${QT_VERSION_MAJOR}::Core
+        Qt${QT_VERSION_MAJOR}::Gui
+        # Qt${QT_VERSION_MAJOR}::X11Extras
         KF5::ConfigCore
         KF5::ConfigGui
         KF5::ConfigWidgets
@@ -98,6 +107,18 @@ target_link_libraries (materialdecoration
     PRIVATE
         KDecoration2::KDecoration
 )
+if (Qt6_FOUND)
+    # The QX11Info class has been removed.
+    target_link_libraries (materialdecoration
+        PUBLIC
+            Qt${QT_VERSION_MAJOR}::GuiPrivate
+    )
+elseif(Qt5_FOUND)
+    target_link_libraries (materialdecoration
+        PUBLIC
+            Qt${QT_VERSION_MAJOR}::X11Extras
+    )
+endif()
 
 install (TARGETS materialdecoration
          DESTINATION ${PLUGIN_INSTALL_DIR}/org.kde.kdecoration2)

src/libdbusmenuqt/CMakeLists.txt

@@ -20,8 +20,8 @@ qt_add_dbus_interface(libdbusmenu_SRCS com.canonical.dbusmenu.xml dbusmenu_inter
 
 add_library(dbusmenuqt STATIC ${libdbusmenu_SRCS})
 target_link_libraries(dbusmenuqt
-    Qt::DBus
-    Qt::Widgets
+    Qt${QT_VERSION_MAJOR}::DBus
+    Qt${QT_VERSION_MAJOR}::Widgets
 )
 
 add_subdirectory(test)