Fix:

error: call to undeclared library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Wimplicit-function-declaration]

https://github.com/uniconproject/unicon/commit/8ccd0c6a73209274b736339af1e65ec4208ee638
--- aclocal.m4.orig	2020-12-20 23:08:43.000000000 -0600
+++ aclocal.m4	2026-03-30 02:45:29.000000000 -0500
@@ -456,7 +456,7 @@
    [
     AC_TRY_LINK(dnl
     [/* no includes */],
-    [ extern long int $ac_global;  exit((int)$ac_global)],
+    [ extern long int $ac_global; return (int)$ac_global],
     eval "ac_cv_global_${ac_global}=yes",
     eval "ac_cv_global_${ac_global}=no"
     )