configure.in: use ac_cv_func_vfork

* configure.in (ac_cv_func_vfork): ac_cv_func_vfork_works is
  overridden always.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47442 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -963,7 +963,7 @@ AS_CASE(["$target_os"],
 		ac_cv_type_getgroups=gid_t # getgroups() on Rosetta fills garbage
 		ac_cv_lib_crypt_crypt=no
 		ac_cv_func_fdatasync=no # Mac OS X wrongly reports it has fdatasync()
-		ac_cv_func_vfork_works=no
+		ac_cv_func_vfork=no
                 AC_CACHE_CHECK(for broken crypt with 8bit chars, rb_cv_broken_crypt,
                     [AC_TRY_RUN([
 #include <stdio.h>