glx/dri2: Fix debug build with old dri2proto.

Apparently the higher compiler optimization level in non-debug builds was
eliminating the unused functions referencing the unresolved DRI2 symbols...
diff --git a/src/glx/dri2_glx.c b/src/glx/dri2_glx.c
index 5b0f335..14788b8 100644
--- a/src/glx/dri2_glx.c
+++ b/src/glx/dri2_glx.c
@@ -202,6 +202,8 @@
    return &pdraw->base;
 }
 
+#ifdef X_DRI2GetMSC
+
 static int
 dri2DrawableGetMSC(__GLXscreenConfigs *psc, __GLXDRIdrawable *pdraw,
 		   int64_t *ust, int64_t *msc, int64_t *sbc)
@@ -209,6 +211,11 @@
    return DRI2GetMSC(psc->dpy, pdraw->xDrawable, ust, msc, sbc);
 }
 
+#endif
+
+
+#ifdef X_DRI2WaitMSC
+
 static int
 dri2WaitForMSC(__GLXDRIdrawable *pdraw, int64_t target_msc, int64_t divisor,
 	       int64_t remainder, int64_t *ust, int64_t *msc, int64_t *sbc)
@@ -225,6 +232,8 @@
 		      sbc);
 }
 
+#endif /* X_DRI2WaitMSC */
+
 static void
 dri2CopySubBuffer(__GLXDRIdrawable *pdraw, int x, int y, int width, int height)
 {
@@ -448,6 +457,8 @@
    return pdraw->buffers;
 }
 
+#ifdef X_DRI2SwapInterval
+
 static void
 dri2SetSwapInterval(__GLXDRIdrawable *pdraw, int interval)
 {
@@ -465,6 +476,8 @@
   return priv->swap_interval;
 }
 
+#endif /* X_DRI2SwapInterval */
+
 static const __DRIdri2LoaderExtension dri2LoaderExtension = {
    {__DRI_DRI2_LOADER, __DRI_DRI2_LOADER_VERSION},
    dri2GetBuffers,