configure: Disable xvmc by default

The xvmc unit tests are failing on r300g and r600g.

Reviewed-by: Vinson Lee <vlee@freedesktop.org>
diff --git a/configure.ac b/configure.ac
index 4b55140..475bc6c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -583,7 +583,7 @@
    [AS_HELP_STRING([--enable-xvmc],
          [enable xvmc library @<:@default=auto@:>@])],
    [enable_xvmc="$enableval"],
-   [enable_xvmc=auto])
+   [enable_xvmc=no])
 AC_ARG_ENABLE([vdpau],
    [AS_HELP_STRING([--enable-vdpau],
          [enable vdpau library @<:@default=auto@:>@])],