tk: fix build on darwin

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.

pkgs/development/libraries/tk/generic.nix

@@ -19,7 +19,7 @@ stdenv.mkDerivation {
   postInstall = ''
     ln -s $out/bin/wish* $out/bin/wish
     cp ../{unix,generic}/*.h $out/include
-    ln -s $out/lib/libtk${tcl.release}.so $out/lib/libtk.so
+    ln -s $out/lib/libtk${tcl.release}${stdenv.hostPlatform.extensions.sharedLibrary} $out/lib/libtk${stdenv.hostPlatform.extensions.sharedLibrary}
   ''
   + stdenv.lib.optionalString (stdenv.isDarwin) ''
     cp ../macosx/*.h $out/include