Rework the dl* Darwin compatibility code so it uses the real functions when

Rework the dl* Darwin compatibility code so it uses the real functions when
available.  If compiled to allow for pre-10.3 systems it will do the check
at runtime and the compatibility functions will be compiled but likely
not used.  If compiled with 10.3 or higher as the minimum version, the
compatibility functions will not be compiled and the library will work
identically as it does on all other UNIX.

Even if the compatibility code is compiled in, the function names are now
prefixed with wx_darwin_ and are marked static so they don't make it into
the exported symbols, even in their C++-mangled form.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@51898 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775