deprecated qmake build system / removed Qt5 fallback when using `USE_QT6=On` (#5271)

QT 5 is now completely EOL and since we never supported Qt6 in qmake
which also has various shortcomings, it is time to at least deprecate it
and direct users to CMake instead.

cmake/findDependencies.cmake

@@ -7,18 +7,10 @@ if (BUILD_GUI)
         list(APPEND qt_components Test)
     endif()
     if (USE_QT6)
-        find_package(Qt6 COMPONENTS ${qt_components} QUIET)
-    endif()
-    if (NOT Qt6Core_FOUND)
-        find_package(Qt5 COMPONENTS ${qt_components} REQUIRED)
-        set(QT_VERSION "${Qt5Core_VERSION_STRING}")
-    else()
-        # detect faulty Qt6 installation by jurplel/install-qt-action@v2
-        if (WITH_QCHART AND NOT Qt6Charts_FOUND)
-            message(FATAL_ERROR "QtCharts not found")
-        endif()
+        find_package(Qt6 COMPONENTS ${qt_components} REQUIRED)
         set(QT_VERSION "${Qt6Core_VERSION_STRING}")
         if (NOT QT_VERSION)
+            # TODO: how to get the actual version?
             message(WARNING "'Qt6Core_VERSION_STRING' is not set - using 6.0.0 as fallback")
             set(QT_VERSION "6.0.0")
         endif()
@@ -31,6 +23,9 @@ if (BUILD_GUI)
             # https://bugreports.qt.io/browse/QTBUG-86395
             set(DISABLE_CRTDBG_MAP_ALLOC ON)
         endif()
+    else()
+        find_package(Qt5 COMPONENTS ${qt_components} REQUIRED)
+        set(QT_VERSION "${Qt5Core_VERSION_STRING}")
     endif()
 endif()
 

gui/gui.pro

@@ -1,4 +1,7 @@
 lessThan(QT_MAJOR_VERSION, 5): error(requires >= Qt 5 (You used: $$QT_VERSION))
+greaterThan(QT_MAJOR_VERSION, 5): error(Qt 6 is not supported via qmake - please use CMake instead)
+
+message("Building the GUI via qmake is deprecated and will be removed in a future release. Please use CMake instead.")
 
 TEMPLATE = app
 TARGET = cppcheck-gui

readme.md

@@ -86,6 +86,8 @@ cmake --build . --config RelWithDebInfo
 
 ### qmake
 
+NOTE: This has been deprecated and will be removed in a future version. Please use CMake instead.
+
 You can use the gui/gui.pro file to build the GUI.
 
 ```shell

releasenotes.txt

@@ -8,4 +8,8 @@ GUI:
 
 Changed interface:
 
+Deprecations:
+- The qmake build system has been deprecated and will be removed in a future version.
+
 Other:
+- "USE_QT6=On" will no longer fallback to Qt5 when Qt6 is not found.