fix: (cmake) Correctly set QT_VERSION_MAJOR and find Qt6 private modules

The previous CMake logic used `find_package(QT NAMES Qt6 ...)` followed by
`if(Qt6_VERSION VERSION_GREATER_EQUAL 6.10)` to conditionally find private
modules, but `Qt6_VERSION` is not set by this call. As a result, Qt6 private
modules (CorePrivate, GuiPrivate, WidgetsPrivate) were never found, causing
link errors on targets depending on them.

This patch updates the logic to:
- Set `QT_VERSION_MAJOR` and `QT_VERSION_MINOR` based on the public Qt6/Qt5
  version variables, preserving existing version checks in the project.
- Conditionally find Qt6 private modules using `find_package(Qt6 CONFIG REQUIRED
  COMPONENTS CorePrivate GuiPrivate WidgetsPrivate)` when Qt6 >= 6.10.
- Maintain compatibility with Qt5 builds.

This ensures private modules are correctly linked when available and
prevents undefined reference errors.

Author: hillwoodroc

CMakeLists.txt

@@ -21,12 +21,20 @@ option(ENABLE_COV "Enable code coverage" OFF)
 if(PROJECT_VERSION_MAJOR EQUAL 6)
   set(VERSION_SUFFIX 6)
   find_package(QT NAMES Qt6 REQUIRED COMPONENTS Core)
-  if(Qt6_VERSION VERSION_GREATER_EQUAL 6.10)
-    find_package(Qt6 COMPONENTS CorePrivate GuiPrivate WidgetsPrivate REQUIRED)
+  if(NOT DEFINED QT_VERSION_MAJOR)
+    set(QT_VERSION_MAJOR ${Qt6_VERSION_MAJOR})
+    set(QT_VERSION_MINOR ${Qt6_VERSION_MINOR})
+  endif()
+  if(QT_VERSION_MAJOR GREATER 6 OR (QT_VERSION_MAJOR EQUAL 6 AND QT_VERSION_MINOR GREATER_EQUAL 10))
+    find_package(Qt6 CONFIG REQUIRED COMPONENTS CorePrivate GuiPrivate WidgetsPrivate)
   endif()
 else()
   set(VERSION_SUFFIX)
   find_package(QT NAMES Qt5 REQUIRED COMPONENTS Core)
+  if(NOT DEFINED QT_VERSION_MAJOR)
+    set(QT_VERSION_MAJOR ${Qt5_VERSION_MAJOR})
+    set(QT_VERSION_MINOR ${Qt5_VERSION_MINOR})
+  endif()
 endif()
 
 set(CMAKE_CXX_STANDARD 17)