configure: Improve OpenGL dependency detections

This has the following visible changes:

- GBM is required only for OpenGL dma-buf.
- X11 is explicitly required by gtk-egl.
- EGL is now mandatory for the OpenGL displays.

The last one needs some detailed description. Before this change,
EGL was tested only for OpenGL dma-buf with the check of
EGL_MESA_image_dma_buf_export. However, all of the OpenGL
displays depend on EGL and EGL_MESA_image_dma_buf_export is always
defined by epoxy's EGL interface.
Therefore, it makes more sense to always check the presence of EGL
and say the OpenGL displays are available along with OpenGL dma-buf
if it is present.

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20210223060307.87736-1-akihiko.odaki@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/gtk.c

@@ -657,6 +657,8 @@ static const DisplayChangeListenerOps dcl_gl_area_ops = {
     .dpy_has_dmabuf          = gd_has_dmabuf,
 };
 
+#ifdef CONFIG_X11
+
 static const DisplayChangeListenerOps dcl_egl_ops = {
     .dpy_name             = "gtk-egl",
     .dpy_gfx_update       = gd_egl_update,
@@ -679,6 +681,8 @@ static const DisplayChangeListenerOps dcl_egl_ops = {
     .dpy_has_dmabuf          = gd_has_dmabuf,
 };
 
+#endif
+
 #endif /* CONFIG_OPENGL */
 
 /** QEMU Events **/
@@ -797,8 +801,12 @@ static gboolean gd_draw_event(GtkWidget *widget, cairo_t *cr, void *opaque)
             /* invoke render callback please */
             return FALSE;
         } else {
+#ifdef CONFIG_X11
             gd_egl_draw(vc);
             return TRUE;
+#else
+            abort();
+#endif
         }
     }
 #endif
@@ -2031,6 +2039,7 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
                              G_CALLBACK(gl_area_realize), vc);
             vc->gfx.dcl.ops = &dcl_gl_area_ops;
         } else {
+#ifdef CONFIG_X11
             vc->gfx.drawing_area = gtk_drawing_area_new();
             /*
              * gtk_widget_set_double_buffered() was deprecated in 3.14.
@@ -2044,6 +2053,9 @@ static GSList *gd_vc_gfx_init(GtkDisplayState *s, VirtualConsole *vc,
 #pragma GCC diagnostic pop
             vc->gfx.dcl.ops = &dcl_egl_ops;
             vc->gfx.has_dmabuf = qemu_egl_has_dmabuf();
+#else
+            abort();
+#endif
         }
     } else
 #endif
@@ -2354,8 +2366,10 @@ static void early_gtk_display_init(DisplayOptions *opts)
         } else
 #endif
         {
+#ifdef CONFIG_X11
             DisplayGLMode mode = opts->has_gl ? opts->gl : DISPLAYGL_MODE_ON;
             gtk_egl_init(mode);
+#endif
         }
 #endif
     }