Configure script not respecting specified c++ standard

Running `configure --help` shows I can use -c++std option to specify a c++ standard, but passing c++14 as specified in the help text is not respected.

Steps:

• Download Qt 5.15.13 tar.xz file
• Decompress
• `cd qt-everywhere-src-5.15.13`
• Manually apply this patch: https://codereview.qt-project.org/c/qt/qtbase/+/503172
• `export PATH=$PATH:$(pwd)/qtbase/bin`
• `cd qtbase`
• `./configure QMAKE_APPLE_DEVICE_ARCHS="x86_64 arm64" -release -c++std c++14 -opensource -confirm-license -nomake examples -nomake tests`

Actual Result:

• Configure script shows it will use C++ 17 standard instead of the specified C++ 14 standard (see attachment)

Expected result:

• Configure script uses C++ 14