[CMake] Create user and group with CPack

[Kristian]

Hi,

I create a RPM package with CPack. When installing this package on a
target system, it should create an additional user and group.

My current solution is a postinstall script, wich runs at the end of
the installation. There, I added commands to create a user and a group
(groupadd, useradd). But somehow, they aren't created when installing
this RPM package. For testing, I install this RPM package in a docker
container.

I also tried, if these commands (groupadd, useradd) are working on the
docker container and they are, so this shouldn't be a problem.

So, how can I create a user and a group with CPack / CMake?