Don't generate self-references for system include directories.

Targets which link directly to themselves should not result in
generate-time errors (reported by the DAG checker).

Self-links are handled separately with policy CMP0038.
Help: Fix up the VISIBILITY_INLINES_HIDDEN docs.

Resolve some sentence fragments, remove a copy-pasto from other docs
and use a link to the CMAKE_VISIBILITY_INLINES_HIDDEN variable.
Merge topic 'minor-cleanups' into next

8cc3cdb Help: Fix up the VISIBILITY_INLINES_HIDDEN docs.
1396ab8 Don't generate self-references for system include directories.