Restore correct wx-config creation and work around MinGW lack of symlinks.

Restore correct wx-config creation and work around MinGW lack of symlinks.

This undoes the changes of r72205 and r72249 (see #14517) as using relative
paths for wx-config symlink didn't work correctly if non-default values for
$bindir and/or $libdir were used (i.e. if they were not siblings).

Instead, fix the original problem of lack of symlinks when using MinGW
directly: just use "cp -p" if "ln -s" fails. This should work everywhere and
do the right thing.

Closes #15463.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74758 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775