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.
The command has been documented as 'deprecated', but it is not really
slated for removal and can still be used. Clarify this in the
documentation. While at it, revise the documentation to use wording
more consistent with that now in target_link_libraries.
Suggested-by: Christoph Grüninger <foss@grueninger.de>