makefile: fix build with overriden UTILS

[Ecordonnier]

The make build was failing when the list of UTILS didn't contain stdbuf. This fixes this issue by adding feat_externa_libstdbuf only when stdbuf is being compiled.

Fixes #8701

[sylvestre]

maybe add a github action test to make sure we don't regress ?

[codspeed-hq]

CodSpeed Performance Report

Merging #8702 will not alter performance

_{Comparing Ecordonnier:eco/makefile-stdbuf (873c376) with main (ea5c815)}

Summary

✅ 75 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[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)

[github-actions]

GNU testsuite comparison:

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

[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)

[github-actions]

GNU testsuite comparison:

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

[Ecordonnier]

maybe add a github action test to make sure we don't regress ?

Done

[github-actions]

GNU testsuite comparison:

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