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.
