patch 8.2.1119: configure fails with Xcode 12 beta

Problem:    Configure fails with Xcode 12 beta.
Solution:   use "return" instead of "exit()". (Nico Weber, closes #6381)
diff --git a/src/configure.ac b/src/configure.ac
index 0543918..19ce4c7 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -4151,8 +4151,8 @@
 main() {
   uint32_t nr1 = (uint32_t)-1;
   uint32_t nr2 = (uint32_t)0xffffffffUL;
-  if (sizeof(uint32_t) != 4 || nr1 != 0xffffffffUL || nr2 + 1 != 0) exit(1);
-  exit(0);
+  if (sizeof(uint32_t) != 4 || nr1 != 0xffffffffUL || nr2 + 1 != 0) return 1;
+  return 0;
 }],
 AC_MSG_RESULT(ok),
 AC_MSG_ERROR([WRONG!  uint32_t not defined correctly.]),