Convert all instances of XTHREADS to USE_XTHREADS.  This fixes one of
serveral things that are broken when building on a system with X.org 7.0rc0
installed.
diff --git a/src/mesa/glapi/glthread.h b/src/mesa/glapi/glthread.h
index 503bb44..71e5f33 100644
--- a/src/mesa/glapi/glthread.h
+++ b/src/mesa/glapi/glthread.h
@@ -65,7 +65,7 @@
 
 
 #if (defined(PTHREADS) || defined(SOLARIS_THREADS) ||\
-     defined(WIN32_THREADS) || defined(XTHREADS) || defined(BEOS_THREADS)) \
+     defined(WIN32_THREADS) || defined(USE_XTHREADS) || defined(BEOS_THREADS)) \
     && !defined(THREADS)
 # define THREADS
 #endif
@@ -193,7 +193,7 @@
  * XFree86 has its own thread wrapper, Xthreads.h
  * We wrap it again for GL.
  */
-#ifdef XTHREADS
+#ifdef USE_XTHREADS
 #include <X11/Xthreads.h>
 
 typedef struct {
@@ -225,7 +225,7 @@
 #define _glthread_UNLOCK_MUTEX(name) \
    (void) xmutex_unlock(&(name))
 
-#endif /* XTHREADS */
+#endif /* USE_XTHREADS */