iOS builds should not use C++17

./configure -opensource -confirm-license -release -nomake tests -nomake examples -skip qtsvg -skip qtactiveqt -skip qtscript -skip qttools -skip qtxmlpatterns -skip qttranslations -skip qtdoc -skip qtlocation -skip qtsensors -skip qtconnectivity -skip qtwayland -skip qt3d -skip qtimageformats -skip qtserialbus -skip qtserialport -skip qtx11extras -skip qtmacextras -skip qtwinextras -skip qtandroidextras -skip qtwebsockets -skip qtwebchannel -skip qtwebengine -skip qtcanvas3d -skip qtwebview -skip qtpurchasing -skip qtcharts -skip qtdatavis3d -skip qtvirtualkeyboard -skip qtgamepad -skip qtscxml -skip qtspeech -skip qtnetworkauth -skip qtremoteobjects -prefix /Users/martin/dev/lib/build/qt/5.9.8/ios -xplatform macx-ios-clang

/Users/martin/Downloads/build/qt-everywhere-opensource-src-5.9.8/qtdeclarative/src/quick/items/qquickgenericshadereffect.cpp
/Users/martin/Downloads/build/qt-everywhere-opensource-src-5.9.8/qtdeclarative/src/quick/items/qquickshadereffect.cpp:511:20: error: aligned allocation function of type
      'void *(std::size_t, std::align_val_t)' is only available on iOS 11 or newer [-Waligned-allocation-unavailable]
        m_glImpl = new QQuickOpenGLShaderEffect(this, this);
                   ^
/Users/martin/Downloads/build/qt-everywhere-opensource-src-5.9.8/qtdeclarative/src/quick/items/qquickshadereffect.cpp:511:20: note: if you supply your own aligned allocation functions, use
      -Wno-aligned-allocation-unavailable to silence this diagnostic
1 error generated.
make[3]: *** [.obj/qquickshadereffect.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [sub-quick-make_first-ordered] Error 2
make[1]: *** [sub-src-make_first] Error 2
make: *** [module-qtdeclarative-make_first] Error 2

-c++std c++14