Fix some typical C99 compatibility issues in old configure scripts.

diff --git a/src/sed/config/getline.m4 b/src/sed/config/getline.m4
index ff8b5f4b..52d2e3e7 100644
--- a/src/sed/config/getline.m4
+++ b/src/sed/config/getline.m4
@@ -27,7 +27,7 @@ AC_DEFUN([AM_FUNC_GETLINE],
       if (!in)
 	return 1;
       len = getline (&line, &siz, in);
-      exit ((len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1);
+      return (len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1;
     }
     ], am_cv_func_working_getline=yes dnl The library version works.
     , am_cv_func_working_getline=no dnl The library version does NOT work.
diff --git a/src/sed/configure b/src/sed/configure
index 0b01618f..b1172113 100755
--- a/src/sed/configure
+++ b/src/sed/configure
@@ -2593,7 +2593,7 @@ if { (eval echo "$as_me:$LINENO: \"$ac_compile\"") >&5
   echo "$as_me:$LINENO: \$? = $ac_status" >&5
   (exit $ac_status); }; }; then
   for ac_declaration in \
-   '' \
+   '#include <stdlib.h>' \
    'extern "C" void std::exit (int) throw (); using std::exit;' \
    'extern "C" void std::exit (int); using std::exit;' \
    'extern "C" void exit (int) throw ();' \
@@ -3378,8 +3378,8 @@ main ()
   for (i = 0; i < 256; i++)
     if (XOR (islower (i), ISLOWER (i))
 	|| toupper (i) != TOUPPER (i))
-      exit(2);
-  exit (0);
+      return 2;
+  return 0;
 }
 _ACEOF
 rm -f conftest$ac_exeext
@@ -5747,7 +5747,7 @@ cat >>conftest.$ac_ext <<_ACEOF
       if (!in)
 	return 1;
       len = getline (&line, &siz, in);
-      exit ((len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1);
+      return (len == 4 && line && strcmp (line, "foo\n") == 0) ? 0 : 1;
     }
 
 _ACEOF