refactor: query Qt variables using ECM

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>

cmake/QtVersionOption.cmake

@@ -0,0 +1,38 @@
+#.rst:
+# QtVersionOption
+# ---------------
+#
+# Adds a build option to select the major Qt version if necessary,
+# that is, if the major Qt version has not yet been determined otherwise
+# (e.g. by a corresponding find_package() call).
+#
+# This module is typically included by other modules requiring knowledge
+# about the major Qt version.
+#
+# ``QT_MAJOR_VERSION`` is defined to either be "5" or "6".
+#
+#
+# Since 5.82.0.
+
+#=============================================================================
+# SPDX-FileCopyrightText: 2021 Volker Krause <vkrause@kde.org>
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+if (DEFINED QT_MAJOR_VERSION)
+    return()
+endif()
+
+if (TARGET Qt5::Core)
+    set(QT_MAJOR_VERSION 5)
+elseif (TARGET Qt6::Core)
+    set(QT_MAJOR_VERSION 6)
+else()
+    option(BUILD_WITH_QT6 "Build against Qt 6" OFF)
+
+    if (BUILD_WITH_QT6)
+        set(QT_MAJOR_VERSION 6)
+    else()
+        set(QT_MAJOR_VERSION 5)
+    endif()
+endif()