pkg/60241: qt6-qtbase: Build fails on macOS with error "implicitly declaring library function '__yield' with type 'void ()'"

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>Number:         60241
>Category:       pkg
>Synopsis:       qt6-qtbase: Build fails on macOS with error "implicitly declaring library function '__yield' with type 'void ()'"
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 08 13:40:00 +0000 2026
>Originator:     Daniel Bradbury
>Release:        pkgsrc 2026Q1
>Organization:
>Environment:
Darwin danmbp 25.4.0 Darwin Kernel Version 25.4.0: Thu Mar 19 19:33:43 PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T8142 arm64
>Description:
When attempting to build the qt6-qtbase package on macOS, the build process fails very shortly after it starts with several build failures like this, all with the same error message:

```
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/global/qalloc.cpp:6:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qalgorithms.h:1:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/tools/qalgorithms.h:11:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qglobal.h:1:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/global/qglobal.h:51:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qatomic.h:1:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/thread/qatomic.h:9:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qbasicatomic.h:1:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/thread/qbasicatomic.h:9:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qatomic_cxx11.h:1:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/thread/qatomic_cxx11.h:10:
In file included from /opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/include/QtCore/qyieldcpu.h:1:
/opt/pkgsrc/x11/qt6-qtbase/work/qtbase-everywhere-src-6.10.2/src/corelib/thread/qyieldcpu.h:37:5: error: implicitly declaring library function '__yield' with type 'void ()'
      [-Werror,-Wimplicit-function-declaration]
   37 |     __yield();              // Generic
      |     ^
```
>How-To-Repeat:
Attempt to build the `qt6-qtbase` package on macOS on the pkgsrc 2026Q1 branch.
>Fix: