Move HAVE_WCSXXX from wx/osx/config_xcode.h to wx/osx/chkconf.h.

Move HAVE_WCSXXX from wx/osx/config_xcode.h to wx/osx/chkconf.h.

The latter file is also used when building from the command line while the
former is only used in Xcode build, as its name indicates (which didn't help
me to notice it, however).

Also restore the test for wcsftime() in configure as it is available even on
older systems but do not test for strnlen() -- as it is not.

These changes amend those of r74523.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74534 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775