Use relative path for wx-config symlink in "make install".

This fixes "make DESTDIR=..." Broken by r71050 but still allows "make
DESTDIR=... install" to work even under MinGW where "ln -s" is mapped to "cp"
and so doesn't work with non-existing first argument.

Closes #14517.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72205 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775