amazing how we managed to do without it for so long. ^^
the name is intentionally somewhat obscure to avoid clashes, and some
namespacing is good anyway.
[ChangeLog][qmake] Added $$num_add() function.
Change-Id: Ib7648b1f425ef096a87b51f158d0f1409e5c4daa
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>