Introduce `CMD_STRICT` to act as a parent for warnings about allowed but
disrecommended usage. Introduce a warning that discourages use of
non-target directives (which modify the build environment for the entire
[sub]tree). Add notes to the official documentation recommending use of
target-specific alternatives. This has been accepted wisdom for quite
some time, but only some of the affected commands made any mention of
this in the documentation.
This family enable to manage link flags
Three new properties:
* directory property: LINK_OPTIONS
* target properties: LINK_OPTIONS and INTERFACE_LINK_OPTIONS
Two new commands
* add_link_options(): to populate directory property
* target_link_options(): to populate target properties
Fixes: #16543