Cater to upstream changes in eglplatform.h

EGL_NO_X11 has been replaced with USE_X11, which breaks all existing
code out there including Qt:
KhronosGroup/EGL-Registry#130

Fix this by defining USE_X11 wherever we do not define EGL_NO_X11.

This is a manual backport/rewrite to Qt 5.15 of
https://codereview.qt-project.org/c/qt/qtbase/+/371780 (Targetting Qt6)

Fixes: QTBUG-96392
Change-Id: I86494881329e5baafc6a07b687b8ea6d84c827d5

Author: keithel-qt

src/gui/configure.json

@@ -834,7 +834,8 @@
                     "// embedded devices, are not intended to be used together with X. EGL support",
                     "// has to be disabled in plugins like xcb in this case since the native display,",
                     "// window and pixmap types will be different than what an X-based platform",
-                    "// plugin would expect."
+                    "// plugin would expect.",
+                    "#define USE_X11"
                 ],
                 "include": [ "EGL/egl.h", "X11/Xlib.h" ],
                 "main": [

src/platformsupport/eglconvenience/qt_egl_p.h

@@ -61,6 +61,10 @@
 # if !defined(Q_OS_INTEGRITY)
 #  define WIN_INTERFACE_CUSTOM   // NV
 # endif // Q_OS_INTEGRITY
+#else // QT_EGL_NO_X11
+// if one has an eglplatform.h with https://github.com/KhronosGroup/EGL-Registry/pull/130
+// that needs USE_X11 to be defined.
+# define USE_X11
 #endif  // QT_EGL_NO_X11
 
 #ifdef QT_EGL_WAYLAND