Compile our own gwen-gtk3 if the supplied one is 4.20.0.

This is a bit brittle because it depends on Martin Preuss applying our
patch (already applied to borrowed/gwen-gtk3) before releasing 4.20.1,
but creating a proper test for the changes proved to be too difficult.

CMakeLists.txt

@@ -352,9 +352,9 @@ IF (WITH_AQBANKING)
       SET(HAVE_KTOBLZCHECK_H 1)
     ENDIF(KTOBLZCHECK_FOUND)
     GNC_PKG_CHECK_MODULES (GWEN_GTK3 gwengui-gtk3)
-    IF(GWEN_GTK3_FOUND)
+    IF(GWEN_GTK3_FOUND AND GWEN_GTK3_VERSION VERSION_GREATER "4.20.0")
       SET(HAVE_GWEN_GTK3 1 CACHE BOOL "True if gwen-gtk3.pc exists")
-    ENDIF(GWEN_GTK3_FOUND)
+    ENDIF()
   ENDIF(WITH_GNUCASH)
 ENDIF (WITH_AQBANKING)