Fix installed wx-config broken by r72205.

Fix installed wx-config broken by r72205.

In an attempt to fix DESTDIR support on platforms without symbolic
links, this commit completely broke the wx-config symlink created by
"make install" everywhere (unless you were insane enough to use
"--prefix=/", that is): $(libdir) is absolute path and so using
../$(libdir) has no chance of working.

Fixed by using `basename $(libdir)`. This is still unsatisfactory,
because it assumes that $(libdir) and $(bindir) are at the same level in
the filesystem -- which, while usually true, is by no means guaranteed.
But at least this works most of the time.

See #14517.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72249 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775