We don't have our own dlxxx() implementations under Darwin since 76c5594
(Remove our own dlxxx() functions emulation for OS X <= 10.3.,
2013-10-17).
wxHAVE_DYNLIB_ERROR is reduced to being the same HAVE_DLERROR, so use
the latter one instead.
Closes https://github.com/wxWidgets/wxWidgets/pull/1826