target_compile_options: Add syntax to specify shell strings

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

1 files changed, 6 insertions, 0 deletions

diff --git a/Help/release/dev/compile-options-shell.rst b/Help/release/dev/compile-options-shell.rst
new file mode 100644
index 0000000..3f83e0c
--- /dev/null
+++ b/Help/release/dev/compile-options-shell.rst
@@ -0,0 +1,6 @@
+compile-options-shell
+---------------------
+
+* :command:`target_compile_options` and :command:`add_compile_options`
+  commands gained a ``SHELL:`` prefix to specify a group of related
+  options using shell-like quoting.