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-egl.c

@@ -208,7 +208,7 @@ void gd_egl_scanout_texture(DisplayChangeListener *dcl,
 void gd_egl_scanout_dmabuf(DisplayChangeListener *dcl,
                            QemuDmaBuf *dmabuf)
 {
-#ifdef CONFIG_OPENGL_DMABUF
+#ifdef CONFIG_GBM
     egl_dmabuf_import_texture(dmabuf);
     if (!dmabuf->texture) {
         return;
@@ -224,7 +224,7 @@ void gd_egl_cursor_dmabuf(DisplayChangeListener *dcl,
                           QemuDmaBuf *dmabuf, bool have_hot,
                           uint32_t hot_x, uint32_t hot_y)
 {
-#ifdef CONFIG_OPENGL_DMABUF
+#ifdef CONFIG_GBM
     VirtualConsole *vc = container_of(dcl, VirtualConsole, gfx.dcl);
 
     if (dmabuf) {
@@ -252,7 +252,7 @@ void gd_egl_cursor_position(DisplayChangeListener *dcl,
 void gd_egl_release_dmabuf(DisplayChangeListener *dcl,
                            QemuDmaBuf *dmabuf)
 {
-#ifdef CONFIG_OPENGL_DMABUF
+#ifdef CONFIG_GBM
     egl_dmabuf_release_texture(dmabuf);
 #endif
 }