-g4 was deprecated in Emscripten 2.0.17, and since we are well past
that version we can safely use -gsource-map instead.
Change-Id: I497155619d6320661f6be0e220b52fa7d6ca0b8e
Reviewed-by: David Skoland <david.skoland@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>