Replace deprecated Q_OS_MACX with Q_OS_MACOS

[softins]

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
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[ann0see]

Same comment as in other PR: this will drop support for old Qt builds and should at least be mentioned in the change log

[softins]

Same comment as in other PR: this will drop support for old Qt builds and should at least be mentioned in the change log

OK, I've added that to the CHANGELOG entry in the initial description. I doubt anybody is still building with a version of Qt older than 5.7.1. CentOS 7 uses 5.9.7, Ubuntu 18.04 uses 5.9.5, Debian 10 uses 5.11.3.

[ann0see]

I changed the changelog entry slightly.