compilers: Add optimization=plain option

https://github.com/mesonbuild/meson/pull/9287 changed the `optimization=0`
to pass `-O0` to the compiler. This change is reasonable by itself
but unfortunately, it breaks `buildtype=plain`, which promises
that “no extra build flags are used”.

`buildtype=plain` is important for distros like NixOS,
which manage compiler flags for optimization and hardening
themselves.

Let’s introduce a new optimization level that does nothing
and set it as the default for `buildtype=plain`.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/markdown/snippets/add_optimization_plain_option.md b/docs/markdown/snippets/add_optimization_plain_option.md
new file mode 100644
index 0000000..67910df
--- /dev/null
+++ b/docs/markdown/snippets/add_optimization_plain_option.md
@@ -0,0 +1,10 @@
+## Add `optimization` `plain` option
+
+The `optimization` built-in option now accepts `plain` value,
+which will not set any optimization flags. This is now the default
+value of the flag for `buildtype=plain`, which is useful for distros,
+that set the optimization and hardening flags by other means.
+
+If you are using the value of `get_option('optimization')` in your
+Meson scripts, make sure you are not making assumptions about it,
+such as that the value can be passed to a compiler in `-O` flag.