The "magic" symbol was QString::fromAscii, which might be fully inlined
in Qt 5. Use QString::fromAscii_helper instead, and also make the detection
a bit more robust in case the magic symbol is not found.

Change-Id: I99e61797356a0d90467d32e54acc2d2a104d609a
Reviewed-by: hjk <qthjk@ovi.com>