Short description of changes

This PR replaces all instances of Q_OS_MACX with Q_OS_MACOS

CHANGELOG: Build: Replace deprecated Q_OS_MACX with Q_OS_MACOS (Jamulus now requires building with Qt >= 5.7.1)

Context: Fixes an issue?

The #define of Q_OS_MACOS was introduced in Qt 5.7.1, to replace Q_OS_MACX. The latter was still defined for backward-compatibility. Starting with Qt 6.5.2, using Q_OS_MACX produces a deprecation warning, for example:

In file included from ../src/settings.cpp:25:
In file included from ../src/settings.h:36:
../src/client.h:47:20: warning: macro 'Q_OS_MACX' has been marked as deprecated: use Q_OS_MACOS instead [-Wdeprecated-pragma]
#    if ( defined( Q_OS_MACX ) ) && !defined( JACK_REPLACES_COREAUDIO )
                   ^
/usr/local/opt/qt/6.6.1/macos/lib/QtCore.framework/Headers/qsystemdetection.h:167:65: note: macro marked 'deprecated' here
#    pragma clang deprecated(Q_OS_MACX, "use Q_OS_MACOS instead")
                                                                ^
1 warning generated.

A similar change was made in Qt itself at https://codereview.qt-project.org/c/qt/qtbase/+/307095/2

Does this change need documentation? What needs to be documented and how?

Probably not.

Status of this Pull Request

Ready to merge,

What is missing until this pull request can be merged?

Just review

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
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