If xcrun prints warnings or errors to stderr while determining the SDK
version, they become part of the SDK version string. This then leads to
a qmake error.
Intentionally ignore stderr so that it is not treated as part of the SDK
version.
Pick-to: 5.15 6.2 6.3
Fixes: QTBUG-102066
Change-Id: I023296b430aac1407c970412c5cf1010bd81589b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>