Introduce fallback for general cross compilation

Introduce fallback for general cross compilation

The current approach of forcing people to pass the fully qualified prefix
of their toolchain to the configure script is verbose and something of a
chore for people who use the same toolchain to target several devices.

This allows you to set a single toolchain for use with all Qt targets via:

qmake -set CROSS_COMPILE foo

You can still explicitly override this toolchain, as originally mandated,
with the configure time device-option arguments.

Change-Id: Ibd3d940bb08fa09499533f9c661557e337a8421a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>