Avoid implicit int because it was removed from the language. Future
compilers will not support implicit ints by default, leading to
compilation failures.

Submitted upstream: <https://github.com/garrigue/labltk/pull/21>

diff --git a/config/auto-aux/tclversion.c b/config/auto-aux/tclversion.c
index 4608ee9f91a51e88..4c703c58284f3aa3 100644
--- a/config/auto-aux/tclversion.c
+++ b/config/auto-aux/tclversion.c
@@ -18,7 +18,9 @@
 #include <tcl.h>
 #include <tk.h>
 
-main ()
+int
+main (void)
 {
     puts(TCL_VERSION);
+    return 0;
 }