Another implicit declaration of exit in the -DXTHREADS check, which
will go wrong with future compilers that do not support implicit
function declarations.

diff --git a/acinclude.m4 b/acinclude.m4
index 61706f3e63d94d8a..88edfce3d169a267 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -22,9 +22,9 @@ int main() {
 Boolean brc;
 brc=XtToolkitThreadInitialize();
 if (True==brc)
-  exit(0);
+  return 0;
 else
-  exit(1);
+  return 1;
 }
 ],
 lt_cv_xthreads=yes,