python-pyqt: fix PyQt_qreal_double enabling/disabling

fix :
http://autobuild.buildroot.net/results/6a2/6a28855c11b321ce8ceaf0acdd5395738af931fd/
http://autobuild.buildroot.net/results/91a/91a3641d1e4126475bbca0d3c779582832f6db91/
http://autobuild.buildroot.net/results/b10/b10fa70c199fc2de405068fea1eac80c29577747/
http://autobuild.buildroot.net/results/b53/b5334ac80afb58a19bd40b7c0b18378d75bc8fc7/
http://autobuild.buildroot.net/results/01f/01fa1319e467c3c2410fd462dc40c18b82dc8246/
http://autobuild.buildroot.net/results/2e6/2e6cae93ab860175c405e440bc7e24334b5b14e8/



According to configure-ng.py, PyQt_qreal_double must be disabled on
ARM target and when QT_NO_FPU is set.

[Thomas: refactor code using a hidden Config.in boolean.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>