Allow to specify both DIRECTORY and TARGET_DIRECTORY at the same time in
`set_source_files_properties()` and `set_property(SOURCE)` commands.
Add test cases and update the documentation.
Fixes: #20932
Both set_source_files_properties() and set_property(SOURCE) now accept
two new optional arguments: DIRECTORY and TARGET_DIRECTORY.
The DIRECTORY option takes a list of relative or absolute paths
pointing to processed source directories (add_subdirectory was
already called on them).
These paths specify directory scopes where the source file properties
will be set. Previously the scope was always the currently processed
source directory.
Similarly TARGET_DIRECTORY takes a list of targets, whose source
directories will be used as the list of scopes where to set the
source file properties.
get_property() and get_source_file_property() also get the same
new arguments, except only one value can be specified instead
of a list.
Fixes: #20128
Revise docs for all "Scripting Commands", except four find_XXX
that use a macro suite of their own.
* Take full advantage of the improved syntax highlighting.
* Make consequential use of <..> placeholders.
* Clarify things here and there in the text.
Specific improvements to some command docs:
* "math": Correct description of novel hexadecimal capability.
* "if", "foreach", "while": Provide link to "endif" etc
* "foreach", "while": Mention "break" and "continue".
* "foreach": Simplify explanation of ``RANGE`` and ``IN`` signatures;
advise against negative arguments or reverse ranges (compare issue #18461)
* "endif", "endfunction" etc: Explain that the argument is optional and
maintained for compatibility only
Since commit v3.1.0-rc1~479^2~1 (Add an "installed file" property
scope, 2014-05-15) the get_property and set_property commands
support an 'INSTALL' scope. Add documentation for this scope.
