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
Options specified via `COMPILE_OPTIONS` and `INTERFACE_COMPILE_OPTIONS`
are deduplicated, but individual options can legitimately be duplicated
when grouped with other options, e.g.
-D A -D B
After deduplication that becomes `-D A B`. Therefore we need a way to
treat groups of options as units during deduplication. A simple approach
is to specify each group as one option, e.g.
"-D A" "-D B"
However, that conflicts with options that legitimately have spaces. To
break this ambiguity, add a `SHELL:` prefix syntax to specify that an
option should be parsed like shell command line arguments after
deduplication, e.g.
"SHELL:-D A" "SHELL:-D B"
These will survive deduplication intact, and then be parsed to produce
`-D A -D B` on the final command line.
Fixes: #15826
Explain that it affects only targets added after the call. Fix the link
to the COMPILE_OPTIONS directory propert documentation. Update the
latter to explain that it is used to initialize the corresponding target
property when a target is created.
Generator expressions are supported in many places and are a distinct
concept worthy of their own manual page. The old builtin documentation
was previously represented by preprocessor macros to generate it into
each place that supports them. Factor out the duplicate content into a
dedicated cmake-generator-expressions manual page and reference it from
each original location.
