qt5: Fix sporadic build failure during top-level parallel build

qt5: Fix sporadic build failure during top-level parallel build

When using top level parallel build, independent qt5 packages may be
built in parallel. Because of their staging dirs being hardlinked, they
all use the same qt.conf file to manipulate during configure, while
another qt5 package might already use it. This leads to weird build failures
because the folders qmake is using are diverted in erratic ways.
Fix this by actually recreating a non-shared qt.conf file for every package.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
Reviewed-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>