Ignoring the entire `/subprojects/` directory prevents the next rule
from including just the Meson wrap files. Instead, ignore all the files
in the directory which allows the intended behavior.
Add Meson wrap manifests for all of our dependencies which can
fallback to a subproject. This makes it easier to build wlroots
on platforms where system packages are outdated. Users can now
opt-in via `meson configure --wrap-mode=default` and Meson will
download and build any missing dependency.
Don't download by default because this can be quite surprising and
undesirable for some users (e.g. if they are just missing some -dev
package).
The previous entries aren't used because the build files are
self-contained in the Meson build directory.
Add subprojects/ because it's common for developers to override
Meson dependencies via subproject symlinks.
