platform.m4: Fix case when we have to source /etc/os-release

[arrowd]

When sourcing this file it may overwrite the VERSION shell variable, which in turn end up being used for @Version@ substitutions in various .in files.

I was lucky to catch this when preparing a FreeBSD port update.
Spec reference: https://www.freedesktop.org/software/systemd/man/latest/os-release.html#VERSION=

[gemini-code-assist]

Code Review

The pull request correctly identifies and fixes an issue where sourcing /etc/os-release can clobber the VERSION variable. However, the fix can be made more robust and portable. My review includes a suggestion to use a subshell to source /etc/os-release, which prevents clobbering of any shell variables, not just VERSION. This approach is cleaner and safer. Additionally, I've corrected a non-POSIX [[ test to a portable [ test, which was a pre-existing bug in this code block.

[gemini-code-assist]

While saving and restoring the VERSION variable works, a more robust and cleaner approach is to source /etc/os-release inside a subshell. This prevents any variables from leaking into the main script's environment, completely avoiding issues with variable clobbering (including VERSION).

Additionally, the [[ ... ]] construct is a bashism and not guaranteed to be available in all POSIX shells that configure scripts may run under. It should be replaced with the portable [ ... ] test.

This suggestion implements both improvements.

        _osname_suse=$(
            . /etc/os-release
            if [ "${ID}" = "suse" ] || [ "${ID_LIKE#*suse*}" != "${ID_LIKE}" ]; then
                echo "suse"
            fi
        )
        if [ "x$_osname_suse" = "xsuse" ]; then
            osname="suse"
        fi