driconf: add a way to override GLX extensions

Support for DRI1 is not implemented, but who still uses that?

v2:
 - Add the option to the list of new features
 - Drop overriding the Xorg-exposed extensions

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Martin Peres <martin.peres@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7212>

src/util/driconf.h

@@ -217,6 +217,12 @@
 #define DRI_CONF_FORCE_GL_NAMES_REUSE(def) \
    DRI_CONF_OPT_B(force_gl_names_reuse, def, "Force GL names reuse")
 
+#define DRI_CONF_GLX_EXTENSION_OVERRIDE(def) \
+   DRI_CONF_OPT_S(glx_extension_override, def, \
+                  "Allow enabling/disabling a list of GLX extensions")
+
+
+
 /**
  * \brief Image quality-related options
  */