Avoid an implicit int for main and calls to the undeclared exit
function.  Improves compatibility with future compilers which
no longer support these C89 language features by default.

Submitted upstream: <https://github.com/AdaCore/florist/pull/10>

diff --git a/aclocal.m4 b/aclocal.m4
index ac47216c492efb0e..54b2e010866946d7 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -254,14 +254,14 @@ AC_DEFUN(AC_POSIX_COMP_OVERLAY,
 AC_MSG_CHECKING(for struct $1 component $2 overlaying $3)
 AC_CACHE_VAL(ac_cv_comp_$2,
 AC_TRY_RUN([#include "confsrc/pconfig.h"
-main()
+int main(void)
 {
   struct $1 x;
   if (&x.$2 == &x.$3) {
     fprintf(stderr,"$2 overlays $3...");
-    exit (1);
+    return 1;
   } else {
-    exit (0);
+    return 0;
   }
 }], eval "ac_cv_comp_$2=yes",
 eval "ac_cv_comp_$2=no", eval "ac_cv_comp_$2=nu"))dnl
diff --git a/configure b/configure
index ebb561bb53bd1199..dcbdae941bc5c52f 100755
--- a/configure
+++ b/configure
@@ -4429,14 +4429,14 @@ fi
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 #include "confsrc/pconfig.h"
-main()
+int main(void)
 {
   struct sigaction x;
   if (&x.sa_sigaction == &x.sa_handler) {
     fprintf(stderr,"sa_sigaction overlays sa_handler...");
-    exit (1);
+    return 1;
   } else {
-    exit (0);
+    return 0;
   }
 }
 _ACEOF