Accept builtin options with -D when making initial meson call

Currently meson only accepts `-Dopt=value` for builtin options when
calling `meson configure` and `--opt=value` for builtin options when
calling `meson` initially. This is a confusing behavior, and users only
get a small warning at the top of a potentially long configuration
summary to catch this.

This has confused end users and developers alike, there are at least 5
duplicates of the bug this fixes, and I have personally been asked about
this more times than I can count. The help documentation doesn't make
it clear that -D cannot be used to set options like prefix and bindir.

This adds support for -D options to the initial meson call, but not --
options to the meson configure call. I think it's better to have one way
to do things, and -- options are kinda one off while -D is used
everywhere else, so lets stick with that.

Related #969

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/markdown/snippets/d-options-for-meson-setup.md b/docs/markdown/snippets/d-options-for-meson-setup.md
new file mode 100644
index 0000000..90dc452
--- /dev/null
+++ b/docs/markdown/snippets/d-options-for-meson-setup.md
@@ -0,0 +1,6 @@
+## Meson now accepts -D for builtin arguments at setup time like configure time
+
+Previously meson required that builtin arguments (like prefix) be passed as
+`--prefix` to `meson` and `-Dprefix` to `meson configure`. Meson now accepts -D
+form like meson configure does. `meson configure` still does not accept the
+`--prefix` form.