GNUmakefile: Fix LN for cygwin

[oech3]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 298 untouched benchmarks
⏩ 48 skipped benchmarks¹

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_{Comparing oech3:patch-6 (edfba4d) with main (209bb97)}

Footnotes

1. 48 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/dd/no-allocate is now passing!