use qmake to get PKG_CONFIG in configure script

use qmake to get PKG_CONFIG in configure script

The value of PKG_CONFIG might depend on device options.
For example, "-device-option PKG_CONFIG" might be used with configure
or a mkspec might prefix PKG_CONFIG with CROSS_COMPILE which is
specified as a device option.

The shell functions of configure for parsing mkspecs do not take
device options into account, but qmake is pretty good at it now.

Change-Id: I1c9558e550c48e8441ebdac34b82066473c2ce3a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>