Fix issue 24. Extract warnings from the build log and unconditionally display them, even if the overall build does not fail.
Build log extended to contain stderr also, not just stdout.
Initial patch supplied by Ashok P. Nadkarni as part of the feature request.
Issue 24 tweak. Suppress output when list of warnings is empty.
Inserted an over-ridable output hook for package messaging.
Modified merged code to use the new print hook.