SDK version detection does not ignore stderr

I'm not certain qmake is the correct component; this is for an issue affecting the build of qtbase, reported downstream at https://trac.macports.org/ticket/63805

At least since Qt 5.12.2, in mkspecs/features/mac/sdk.mk, CHECK_SDK_COMMAND captures the standard error. I believe this is undesirable, and that stderr should instead be ignored (see attached patch), because when a child xcodebuild process emits (seemingly harmless) messages on stderr, the build will fail, incorrectly claiming that the SDK version has changed.

Example on macOS 12 arm64:

cd tools/moc/ && ( test -e Makefile || /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.15.3/bin/qmake -o Makefile /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.15.3/src/tools/moc/moc.pro ) && /Applications/Xcode.app/Contents/Developer/usr/bin/make -f Makefile install

The platform SDK has been changed from version 12.1 to version objc[40447]: Class AMSupportURLConnectionDelegate is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f678) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x107a502c8). One of the two will be used. Which one is undefined. objc[40447]: Class AMSupportURLSession is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f6c8) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x107a50318). One of the two will be used. Which one is undefined. 2022-03-19 18:02:42.366 xcodebuild[40447:106503] XType: com.apple.fonts is not accessible. 2022-03-19 18:02:42.366 xcodebuild[40447:106503] XType: XTFontStaticRegistry is enabled. objc[40448]: Class AMSupportURLConnectionDelegate is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f678) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x1041882c8). One of the two will be used. Which one is undefined. objc[40448]: Class AMSupportURLSession is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f6c8) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x104188318). One of the two will be used. Which one is undefined. 2022-03-19 18:02:42.677 xcodebuild[40448:106520] XType: com.apple.fonts is not accessible. 2022-03-19 18:02:42.677 xcodebuild[40448:106520] XType: XTFontStaticRegistry is enabled. objc[40449]: Class AMSupportURLConnectionDelegate is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f678) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x10457c2c8). One of the two will be used. Which one is undefined. objc[40449]: Class AMSupportURLSession is implemented in both /usr/lib/libamsupport.dylib (0x1fec9f6c8) and /Library/Apple/System/Library/PrivateFrameworks/MobileDevice.framework/Versions/A/MobileDevice (0x10457c318). One of the two will be used. Which one is undefined. 2022-03-19 18:02:42.985 xcodebuild[40449:106534] XType: com.apple.fonts is not accessible. 2022-03-19 18:02:42.985 xcodebuild[40449:106534] XType: XTFontStaticRegistry is enabled. 12.1. This requires a fresh build. Please wipe the build directory completely, including any .qmake.stash and .qmake.cache files generated by qmake.

Example on macOS 12 x86_64:

cd tools/moc/ && ( test -e Makefile || /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.15.3/bin/qmake -o Makefile /opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_aqua_qt5/qt5-qtbase/work/qtbase-everywhere-src-5.15.3/src/tools/moc/moc.pro ) && /Applications/Xcode.app/Contents/Developer/usr/bin/make -f Makefile install

The platform SDK has been changed from version 12.1 to version 2022-03-19 15:24:37.841 xcodebuild[36918:6881554] XType: com.apple.fonts is not accessible. 2022-03-19 15:24:37.841 xcodebuild[36918:6881554] XType: XTFontStaticRegistry is enabled. 2022-03-19 15:24:39.774 xcodebuild[36919:6881571] XType: com.apple.fonts is not accessible. 2022-03-19 15:24:39.774 xcodebuild[36919:6881571] XType: XTFontStaticRegistry is enabled. 12.1. This requires a fresh build. Please wipe the build directory completely, including any .qmake.stash and .qmake.cache files generated by qmake.