Work around a conflict between gcc 8.0 and swig 3.0

The swig 3.0 generated python wrappers trigger a warning converted into an error issued
by gcc 8.0 for using strncpy as follows:
strncpy(buff, "swig_ptr: ", 10);
The reason is this call will truncate the trailing null byte from the string.
This appears to have been fixed in swig master already but that's not released yet
so let disable the warning when compiling the swig wrappers until it is.

libgnucash/app-utils/CMakeLists.txt

@@ -116,6 +116,9 @@ if (WITH_PYTHON)
 
   target_link_libraries(sw_app_utils gncmod-app-utils ${app_utils_ALL_LIBRARIES} ${PYTHON_LIBRARIES})
   set_target_properties(sw_app_utils PROPERTIES PREFIX "_")
+  if (HAVE_STRINGOP_TRUNCATION)
+    target_compile_options(sw_app_utils PRIVATE -Wno-error=stringop-truncation)
+  endif()
 
   target_include_directories (sw_app_utils
     PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}