..as they have their own tools that parses those files, such as
opensysusers[1] that handles sysusers file and opentmpfiles[2] that
handles tmpfiles.d settings

Because of this. Move both sysusers and tmpfiles 'if' function
outside systemd function, allowing independent install
Signed-off-by: Rafli Akmal <thefallenrat@artixlinux.org>

[1] - https://github.com/artix-linux/opensysusers
[2] - https://github.com/OpenRC/opentmpfiles

Changes done by vicentiu@mariadb.org, from original author patch:

Installing sysusers and tmpfiles without checking for systemd existence
means that by default, cmake will ALWAYS install these files. Our
general policy is we do not install things which are not needed.
However, there is a valid use case when these files are useful, as is
described above.

To allow this, provide an extra switch that can be enabled during
configuring by doing -DINSTALL_SYSTEMD_{SYSUSERS|TMPFILES}=True

This will use the default path INSTALL_SYSTEMD_{SYSUSERS|TMPFILES}DIR
fetched from install_layout.cmake for rpm & deb based layouts
respectively, or they must be overriden if the install_layout is
standalone.

Example:

cmake . -DINSTALL_SYSTEMD_SYSUSERS=True -DINSTALL_SYSTEMD_SYSUSERSDIR=/etc/sysusers.d