Merged
Removing the check for libio.h takes care of the warning mentioned in https://lists.gnu.org/archive/html/lilypond-devel/2021-08/msg00073.html .
GitLab 15.0 is launching on May 22! This version brings many exciting improvements, but also removes deprecated features and introduces breaking changes that may impact your workflow. To see what is being deprecated and removed, please visit Breaking changes in 15.0 and Deprecations.
Removing the check for libio.h takes care of the warning mentioned in https://lists.gnu.org/archive/html/lilypond-devel/2021-08/msg00073.html .