2 makefile fixes

[Ecordonnier]

Commit 1:

GNUmakefile: fix installation logic for libstdbuf

It is better to check whether we're actually compiling libstdbuf, rather than to check whether
we're on windows, in order to decide whether libstdbuf should get installed.

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

Commit 2:

GNUmakefile: filter out SKIP_UTILS from UTILS

This fixes this corner-case where SKIP_UTILS was not taken into account in the logic enabling feat_external_libstdbuf:

```
sudo make install SKIP_UTILS=stdbuf
...
error: none of the selected packages contains this feature: feat_external_libstdbuf
```

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

[sylvestre]

could you please add tests to make sure we don't regress? thanks

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[codspeed-hq]

CodSpeed Performance Report

Merging #8782 will not alter performance

_{Comparing Ecordonnier:eco/makefile-fix (159f5c8) with main (16af7d4)}

Summary

✅ 106 untouched
⏩ 73 skipped¹

Footnotes

1. 73 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/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[Ecordonnier]

could you please add tests to make sure we don't regress? thanks

I added some test. Btw the busybox CI job failed because of some network issue and not because of this PR.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)