Fix Qt6 feature checks to ensure the `QT_CONFIG` macro rather than
incorrectly checking for the definitions. `QT_FEATURE_*` are always
defined, and they take values 1 or -1 to indicate whether the feature
is present or not. The `QT_CONFIG` macro wraps checking for that, see:
https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/global/qtconfigmacros.h?id=f9163ae7a8167daded0798654d99a2e3a5aaa2b5#n60
This fixes build failure when qtbase is built without Wayland support:
```
/tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/gui/mediaplayer/libmpv/libmpvwidget.cpp: In member function ‘v
irtual void LibMpvWidget::initializeGL()’:
/tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/gui/mediaplayer/libmpv/libmpvwidget.cpp:123:60: error: ‘QWayla
ndApplication’ is not a member of ‘QNativeInterface’; did you mean ‘QX11Application’?
123 | display.data = qApp->nativeInterface<QNativeInterface::QWaylandApplication>()->display();
| ^~~~~~~~~~~~~~~~~~~
| QX11Application
/tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/gui/mediaplayer/libmpv/libmpvwidget.cpp:123:80: error: no matc
hing function for call to ‘Application::nativeInterface<<expression error> >()’
123 | display.data = qApp->nativeInterface<QNativeInterface::QWaylandApplication>()->display();
In file included from /usr/include/qt6/QtCore/qcoreapplication.h:17,
from /usr/include/qt6/QtCore/QCoreApplication:1,
from /tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/miscellaneous/iofactory.h:8,
from /tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/miscellaneous/application.h:11,
from /tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/gui/mediaplayer/libmpv/libmpvwidget.cpp:
5:
/usr/include/qt6/QtWidgets/qapplication.h:125:5: note: candidate: ‘template<class NativeInterface, class TypeInfo, class BaseType, typename std::enable_if<TypeInfo::isCompatibleWith<QApplication>, bool>::type <anonymous> > NativeInterface* QApplication::nativeInterface() const’
125 | QT_DECLARE_NATIVE_INTERFACE_ACCESSOR(QApplication)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/qt6/QtWidgets/qapplication.h:125:5: note: template argument deduction/substitution failed:
/tmp/portage/net-news/rssguard-4.8.1/work/rssguard-4.8.1/src/librssguard/gui/mediaplayer/libmpv/libmpvwidget.cpp:123:80: error: templat
e argument 1 is invalid
```
Originally reported in https://bugs.gentoo.org/948473.
Starting from version 0.38.0, libmpv stopped using the "libmpv" video
output ("vo" option) by default, which ends up causing mpv to spawn its
own, separate and detached window, instead of just embedding itself
onto RSS Guard's internal media player.
Upstream commit: 0e441525cf
Note: This option is only set when building RSS Guard with the
"MEDIAPLAYER_LIBMPV_OPENGL" option set to "ON", since local testing
showed it's not necessary otherwise.
Co-authored-by: martinrotter <martinrotter@users.noreply.github.com>
