fix: move definitions to inner CMakeLists.txt(s) and correct add_user/add_group

[leogr]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area build
/area libsinsp

What this PR does / why we need it:

While testing the latest libs version on Falco, we noticed that user names were not correctly looked up. Digging into the problem, we discovered that a feature gate 👇 was off when building Falco

libs/userspace/libsinsp/user.cpp

Lines 151 to 163 in bd94f31

	#ifdef HAVE_PWD_H
	if (container_id.empty() && !name)
	{
	// On Host, try to load info from db
	auto *p = getpwuid(uid);
	if (p)
	{
	name = p->pw_name;
	home = p->pw_dir;
	shell = p->pw_shell;
	}
	}
	#endif

After a further investigation, we realized the root problem was that the add_definition was included in CompilerFlags.cmake, which is not used by client projects (like Falco).

This PR moves relevant compiler definitions in cmake module files (intended to be used by client projects).

UPDATE:
We also found a bug in add_user and add_group. Fixed by 7afbad4

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

This hotfix is needed to release Falco 0.32

Does this PR introduce a user-facing change?:

NONE

[deepskyblue86]

CC @gnosek

[FedeDP]

/approve

[FedeDP]

/test build-libs-bundled-deps

[Andreagit97]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, leogr

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