Avoid an implicit declaration of exit.  This prevents compilation
failures in case compilers switch to rejecting implicit function
declarations by default.

Submitted upstream: <https://github.com/oetiker/mrtg/pull/104>

diff --git a/configure b/configure
index 07b0fbda88c3847a..8b87d2750822c02b 100755
--- a/configure
+++ b/configure
@@ -3670,7 +3670,7 @@ else
                 char buffer[1000];
                 sprintf (buffer, "%${format}u", a);
                 sscanf (buffer, "%${format}u", &b);
-                exit (b!=a);
+                return b!=a;
             }
 
 _ACEOF
diff --git a/configure.ac b/configure.ac
index 1416f3c2d6e2f645..939725ccf306fb2f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -37,7 +37,7 @@ AC_CACHE_VAL(mr_cv_long_long_format_specifier,[
                 char buffer[1000];
                 sprintf (buffer, "%${format}u", a);
                 sscanf (buffer, "%${format}u", &b);
-                exit (b!=a);
+                return b!=a;
             }
             ]])],[mr_cv_long_long_format_specifier="%${format}d"
             mr_cv_long_long_format="${format}d"