st/mesa: conditionally enable GL_NV_vdpau_interop

Only enable it when we compile the state tracker as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
diff --git a/configure.ac b/configure.ac
index 3b77030..49be147 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1735,6 +1735,7 @@
 if test "x$enable_vdpau" = xyes; then
     PKG_CHECK_MODULES([VDPAU], [vdpau >= $VDPAU_REQUIRED])
     gallium_st="$gallium_st vdpau"
+    DEFINES="$DEFINES -DHAVE_ST_VDPAU"
 fi
 AM_CONDITIONAL(HAVE_ST_VDPAU, test "x$enable_vdpau" = xyes)