external_project: Add release snippet

docs/markdown/snippets/external_project.md

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+## `unstable_external_project` improvements
+
+- Default arguments are added to `add_project()` in case some tags are not found
+  in `configure_options`: `'--prefix=@PREFIX@'`, `'--libdir=@PREFIX@/@LIBDIR@'`,
+  and `'--includedir=@PREFIX@/@INCLUDEDIR@'`. It was previously considered a fatal
+  error to not specify them.
+- When `verbose` keyword argument is not specified, or is false, command outputs
+  are written on file in `<builddir>/meson-logs/`.
+- `LD` environment variable is not passed any more when running the configure
+  script. It caused issues because Meson uses CC as linker wrapper but autotools
+  expects to real linker (e.g. `/usr/bin/ld`).