xdemos: Fix xdemos which default to using display :0.0 to default to $DISPLAY

Fix xdemos which default to using display :0.0 to default to $DISPLAY,
this is kind of irritating when testing on a display other than :0.0

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
diff --git a/progs/xdemos/glthreads.c b/progs/xdemos/glthreads.c
index b90e6b4..ea54748 100644
--- a/progs/xdemos/glthreads.c
+++ b/progs/xdemos/glthreads.c
@@ -26,7 +26,7 @@
  *  -p                       Open a display connection for each thread
  *  -l                       Enable application-side locking
  *  -n <num threads>         Number of threads to create (default is 2)
- *  -display <display name>  Specify X display (default is :0.0)
+ *  -display <display name>  Specify X display (default is $DISPLAY)
  *  -t                       Use texture mapping
  *
  * Brian Paul  20 July 2000
@@ -573,7 +573,7 @@
 int
 main(int argc, char *argv[])
 {
-   char *displayName = ":0.0";
+   char *displayName = NULL;
    int numThreads = 2;
    Display *dpy = NULL;
    int i;