targets/xvmc-softpipe: Convert to automake

v2: Andreas Boll <andreas.boll.dev@gmail.com>
    - add missing xvmc state tracker to _LIBADD variable

v3: Andreas Boll <andreas.boll.dev@gmail.com>
    - Provide compatibility with scripts for the old Mesa build system
diff --git a/configure.ac b/configure.ac
index d00df42..6a8bedc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2082,6 +2082,7 @@
 		src/gallium/targets/xvmc-nouveau/Makefile
 		src/gallium/targets/xvmc-r300/Makefile
 		src/gallium/targets/xvmc-r600/Makefile
+		src/gallium/targets/xvmc-softpipe/Makefile
 		src/gallium/winsys/Makefile
 		src/gallium/winsys/i915/drm/Makefile
 		src/gallium/winsys/i915/sw/Makefile