Avoid an implicit declaration of exit.  Partial backport of this
upstream commit:

commit a91b95cca2d397c84f8b9bbd602d40209a7092ce
Author: Andy Wingo <wingo@pobox.com>
Date:   Wed Jan 20 21:52:54 2021 +0100

    Update Gnulib to v0.1-4379-g2ef5a9b4b
    
    Also bump required autoconf version to 2.64, as required by Gnulib.

diff --git a/m4/round.m4 b/m4/round.m4
index 45b7df4591e83648..1561225a5355366d 100644
--- a/m4/round.m4
+++ b/m4/round.m4
@@ -57,7 +57,7 @@ int main()
     * (double) (1U << ((DBL_MANT_DIG + 3) / 5))
     * (double) (1U << ((DBL_MANT_DIG + 4) / 5));
   volatile double x = 0.5 - 0.5 / TWO_MANT_DIG;
-  exit (x < 0.5 && round (x) != 0.0);
+  return x < 0.5 && round (x) != 0.0;
 }]])], [gl_cv_func_round_works=yes], [gl_cv_func_round_works=no],
         [case "$host_os" in
            netbsd* | aix*) gl_cv_func_round_works="guessing no";;