re-introduce optional build of DAHDI input plugin

We should only build it when the dahdi/user.h file is actually present.
diff --git a/src/input/dahdi.c b/src/input/dahdi.c
index 4fd4e61..a4dbd03 100644
--- a/src/input/dahdi.c
+++ b/src/input/dahdi.c
@@ -22,6 +22,10 @@
  *
  */
 
+#include "../../config.h"
+
+#ifdef HAVE_DAHDI_USER_H
+
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
@@ -461,3 +465,5 @@
 	/* register the driver with the core */
 	return e1inp_driver_register(&dahdi_driver);
 }
+
+#endif /* HAVE_DAHDI_USER_H */