Add cmake switch to indicate that a Qt5 build is using KDE's Qt 5.15 fork

There's no easy way to detect this automatically, so a manual switch is required

CMakeLists.txt

@@ -425,6 +425,7 @@ if(WITH_CORE)
   else()
     set(QT_MIN_VERSION 5.12.0)
     set(QT_VERSION_BASE "Qt5")
+    set(WITH_QT5_KDE_FORK FALSE CACHE BOOL "Using KDE's Qt 5.15 fork")
   endif()
 
   # Use Qt5SerialPort optionally for GPS

cmake_templates/qgsconfig.h.in

@@ -104,5 +104,7 @@
 
 #cmakedefine ENABLE_TESTS
 
+#cmakedefine WITH_QT5_KDE_FORK
+
 #endif