configure.ac: link Foundation framework

* configure.ac (XLDFLAGS): link against Foundation framework and
  let __NSPlaceholderDictionary initialize, to get rid of crash
  after fork on macOS High Sierra.  [ruby-core:83239] [Bug #14009]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60182 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -4122,8 +4122,8 @@ AS_CASE(["$target_os"],
	],
    [darwin*], [
	RUBY_APPEND_OPTION(CFLAGS, -pipe)
	RUBY_APPEND_OPTION(XLDFLAGS, [-framework CoreFoundation])
	RUBY_APPEND_OPTION(LIBRUBYARG_STATIC, [-framework CoreFoundation])
	RUBY_APPEND_OPTION(XLDFLAGS, [-framework Foundation])
	RUBY_APPEND_OPTION(LIBRUBYARG_STATIC, [-framework Foundation])
	],
    [osf*], [
	AS_IF([test "$GCC" != "yes" ], [