Don't test for wide character functions in configure under OS X.

Don't test for wide character functions in configure under OS X.

The tests for these functions may succeed on the system we're running under
but the functions may not be actually available on the system the application
is going to run under (if it's < 10.7).

So use our own replacements for these functions unless 10.7 was explicitly
chosen as the minimal possible version.

An alternative solution could have been to use weak linking but this is more
difficult and there is no real gain (performance would need to be measured
first).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74523 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775