Installing libtk on macOS 10.15.5 resulted in a following warning:
warning: skipping dangling symlink '/nix/store/3whl4256v6qf15dsm1d5mf0lzhfh6w06-user-environment/lib/libtk.so'
which was caused by the following code in generic.nix of the tk derivation:
ln -s $out/lib/libtk${super.tcl.release}.so $out/lib/libtk.so
Shared libraries on macOS are suffixed `.dylib' not 'so' so
libtk${version}.so does not actually exist. Consequently a symlink from
libtk.dylib is never created and linking against tk fails.
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