Add comment about getting valid Coverity builds

CMakeLists.txt

@@ -75,6 +75,8 @@ add_definitions(
   -DQT_NO_URL_CAST_FROM_STRING
 )
 
+# FIXME: on my system coverity build doesn't work with 17; change to 11 to
+#        get a scan
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)