cygwin: force native symlinks for libraries

This will override the existing winsymlinks setting. nativestrict
will cause ln to fail if it's unable to create a native symlink.
Native symlinks are required for the windows dll loader to find the
libraries.

This script is also used for cross-mingw, but setting CYGWIN
shouldn't cause a problem.

pkgs/build-support/setup-hooks/win-dll-link.sh

···
         local dllPath2
         for dllPath2 in "$dllPath" "$(dirname $(readlink "$dllPath" || echo "$dllPath"))"/*.dll; do
             if [ -e ./"$(basename "$dllPath2")" ]; then continue; fi
-            ln -sr "$dllPath2" .
+            CYGWIN+=\ winsymlinks:nativestrict ln -sr "$dllPath2" .
             linkCount=$(($linkCount+1))
         done
     done