Get rid of QPA_XCB define

Since the platform is selected at run time (e.g. xcb vs. Wayland), it
is not a good idea to decide at compile time what code is run. Another
example is on macOS, where having xcb libraries available will lead to
lauching the Xquartz whenever LyX is running.

With this patch, things are separated:
- the code is run when run-time platform is "xcb".
- the support code is compiled in when xcb header and libraries are
  available.

Fixes ticket #13086.

config/qt.m4

@@ -217,31 +217,15 @@ AC_DEFUN([QT_DO_IT_ALL],
 	     fi;;
 	esac
 
+	dnl Specific support for X11 will be built if these are available
+	AC_CHECK_HEADERS([xcb/xcb.h])
+	AC_CHECK_LIB([xcb], [xcb_send_event])
 
 	save_CPPFLAGS=$CPPFLAGS
 	CPPFLAGS="$save_CPPFLAGS $QT_CORE_INCLUDES"
 	AC_CHECK_HEADER(QtGui/qtgui-config.h,
 	  [lyx_qt_config=QtGui/qtgui-config.h],
 	  [lyx_qt_config=qconfig.h])
-	AC_MSG_CHECKING([whether Qt uses the X Window system])
-	if test x$USE_QT6 = xyes ; then
-	  dnl FIXME: Check whether defining QPA_XCB makes sense with Qt6
-	  AC_PREPROC_IFELSE([AC_LANG_SOURCE([
-	    [#include <$lyx_qt_config>]
-	    [#if !defined(QT_FEATURE_xcb) || QT_FEATURE_xcb < 0]
-	    [#error Fail]
-	    [#endif]])],
-	    [AC_MSG_RESULT(yes)
-	     AC_DEFINE(QPA_XCB, 1, [Define if Qt uses the X Window System])],
-	    [AC_MSG_RESULT(no)])
-	else
-	  AC_EGREP_CPP(xcb,
-	    [#include <$lyx_qt_config>
-	    QT_QPA_DEFAULT_PLATFORM_NAME],
-	    [AC_MSG_RESULT(yes)
-	     AC_DEFINE(QPA_XCB, 1, [Define if Qt uses the X Window System])],
-	    [AC_MSG_RESULT(no)])
-	fi
 	CPPFLAGS=$save_CPPFLAGS
 
 	QT_FIND_TOOL([QT_MOC], [moc])

src/frontends/qt/GuiApplication.cpp

@@ -121,7 +121,7 @@
 #include <QThreadPool>
 #include <QWidget>
 
-#if defined(QPA_XCB)
+#ifdef HAVE_XCB_XCB_H
 #include <xcb/xcb.h>
 #ifdef HAVE_QT5_X11_EXTRAS
 #include <QtX11Extras/QX11Info>
@@ -1157,10 +1157,14 @@ GuiApplication::GuiApplication(int & argc, char ** argv)
 	// Install Qt native translator for GUI elements.
 	installTranslator(&d->qt_trans_);
 
-#ifdef QPA_XCB
+	if (platformName() == "xcb") {
+#if defined(HAVE_XCB_XCB_H) && defined(HAVE_LIBXCB)
 		// Enable reception of XCB events.
 		installNativeEventFilter(this);
+#else
+		LYXERR0("Warning: X11 support is incomplete in this LyX binary.");
 #endif
+	}
 
 	// FIXME: quitOnLastWindowClosed is true by default. We should have a
 	// lyxrc setting for this in order to let the application stay resident.
@@ -1181,13 +1185,13 @@ GuiApplication::GuiApplication(int & argc, char ** argv)
 			this, SLOT(onApplicationStateChanged(Qt::ApplicationState)));
 #endif
 
-#if defined(QPA_XCB)
+	if (platformName() == "xcb") {
 		// doubleClickInterval() is 400 ms on X11 which is just too long.
 		// On Windows and Mac OS X, the operating system's value is used.
 		// On Microsoft Windows, calling this function sets the double
 		// click interval for all applications. So we don't!
 		QApplication::setDoubleClickInterval(300);
-#endif
+	}
 
 	connect(this, SIGNAL(lastWindowClosed()), this, SLOT(onLastWindowClosed()));
 
@@ -3498,7 +3502,7 @@ bool GuiApplication::longOperationStarted() {
 //
 // X11 specific stuff goes here...
 
-#if defined(QPA_XCB)
+#if defined(HAVE_XCB_XCB_H) && defined(HAVE_LIBXCB)
 bool GuiApplication::nativeEventFilter(const QByteArray & eventType,
 				       void * message, QINTPTR *)
 {

src/frontends/qt/GuiApplication.h

@@ -20,7 +20,7 @@
 
 #include <QApplication>
 #include <QList>
-#ifdef QPA_XCB
+#if defined(HAVE_XCB_XCB_H) && defined(HAVE_LIBXCB)
 #include <QAbstractNativeEventFilter>
 #endif
 
@@ -53,7 +53,7 @@ There should be only one instance of this class. No Qt object
 initialisation should be done before the instantiation of this class.
 */
 class GuiApplication : public QApplication, public Application
-#ifdef QPA_XCB
+#if defined(HAVE_XCB_XCB_H) && defined(HAVE_LIBXCB)
 		     , public QAbstractNativeEventFilter
 #endif
 {
@@ -121,7 +121,7 @@ public:
 	//@{
 	bool notify(QObject * receiver, QEvent * event) override;
 	void commitData(QSessionManager & sm);
-#if defined(QPA_XCB)
+#if defined(HAVE_XCB_XCB_H) && defined(HAVE_LIBXCB)
 #if (QT_VERSION < 0x060000)
 #define QINTPTR long
 #else