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/*!
//! [expressing os versions]

    Apple platforms have a built-in way to express the OS versions that an
    application supports, which allows older versions of the platforms to
    automatically display a user friendly error message prompting the user to
    update their OS, as opposed to crashing and displaying a stack trace.

    The main concepts involved in expressing support for a particular range of
    OS versions are:

    \list
        \li \e {Deployment target} specifies the \e hard minimum version of
            \macos or iOS that your application supports.
        \li \e {SDK version} specifies the \e soft maximum version of \macos
            or iOS that your application supports.
    \endlist

    When you develop an application for an Apple platform, you should always
    use the latest version of Xcode and the latest SDK available at the time of
    development. On some platforms, like iOS, you will actually be rejected from
    the App Store if you do not. Therefore, the SDK version is always
    greater than or equal to the deployment target.

    When you develop an application for an Apple platform, you must set the
    deployment target. Various build tools within the Xcode toolchain all have
    a flag which you can use to set this value, including but not limited to
    the compiler and linker. By setting the deployment target value, you are
    explicitly declaring that your application must work on at least that
    version, and will not work with any earlier versions of the OS. It is then
    up to you to ensure that your use of the system APIs matches what you have
    declared. Since the compiler knows what you have declared, it can help in
    enforcing that.

    The SDK version is considered a \e soft maximum version of the OS that an
    application is compatible with in the way that if the application is built
    with an SDK, it will continue to use the behaviors of that SDK even on newer
    OS versions, because the OS checks the binary's load commands and emulates
    backwards compatibility with the older OS. For example, if an application is
    built with the \macos 10.12 SDK, it will continue to use 10.12 behaviors
    even on 10.13 and above.

    However, Mach-O binaries are inherently forward compatible. For example, an
    application built with the iOS 9 SDK will run just fine on iOS 10, but might
    not be opted into whatever behavior changes may have been made to certain
    functionality on the new release, until that application is recompiled
    against that newer SDK.

    The minimum OS version can be expressed to the system by the compiler and
    linker flags that embed it into the Mach-O binary. In addition, the
    \c LSMinimumSystemVersion key must be set in the application's app bundle.
    This value must be equal to the value passed to the compiler and linker,
    because on \macos it will allow the OS to display a user friendly error
    dialog that says the application requires a newer version of the OS as
    opposed to a crash dialog. The \c LSMinimumSystemVersion is also the key
    that the App Store uses to display the required OS version; the compiler and
    linker flags have no power there.

    For the most part, Qt applications will work without problems. For example,
    in qmake, the Qt mkspecs set \l QMAKE_IOS_DEPLOYMENT_TARGET or
    \l QMAKE_MACOSX_DEPLOYMENT_TARGET to the minimum version that Qt itself
    supports. Similarly, in Qbs, the Qt modules set \c cpp.minimumIosVersion,
    \c cpp.minimumMacosVersion, \c cpp.minimumTvosVersion, or
    \c cpp.minimumWatchosVersion to the minimum version that Qt itself supports.

    However, you must take care when you manually set your own target version.
    If you set it to a value higher than what Qt requires and supply your own
    \c Info.plist file, you must add an \c LSMinimumSystemVersion entry to the
    \c Info.plist that matches the value of the deployment target, because the
    OS will use the \c LSMinimumSystemVersion value as the authoritative one.

    If you specify a deployment target value lower than what Qt requires, the
    application will almost certainly crash somewhere in the Qt libraries when
    run on an older version than Qt supports. Therefore, make sure that the
    actual build system code reflects the minimum OS version that is actually
    required.

//! [expressing os versions]
*/
