MDEV-35850 make HOSTNAME a cmake configure variable

[grooverdan]

• The Jira issue number for this PR is: MDEV-35850

Description

As seen with openwrt and some other distros, the
determination of hostname can sometime need alternate commands.

This provides a cmake option HOSTNAME for non-windows machines for the mariadb-install-db and mariadbd-safe scripts and the support-files init scripts..

Release Notes

For packagers, HOSTNAME is now a configure option that can be set to the command used to determine the hostname at runtime.

How can this PR be tested?

cmake -DHOSTNAME="cat /proc/sys/kernel/hostname" and examine scripts/mysql_install_db.sh generated.

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main branch.
• This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.
• This is a new compile feature for the benefit of packages that is exceptionally low risk.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.