Ease use of imsg-compat as meson subproject

[shtrophic]

With this patch applied, meson enjoyers are able to just add subprojects/imsg-compat.wrap to their projects for meson to be able to fallback to a local build instead of searching for a system install of imsg-compat:

[wrap-file]
directory = imsg-compat-8.0.0

source_url = https://github.com/bsd-ac/imsg-compat/archive/refs/tags/8.0.0.tar.gz
source_filename = 8.0.0.tar.gz
surce_hash = ...

Afterwards, submitting to WrapDB might be an option.

Related documentation:

• https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonoverride_dependency
• https://mesonbuild.com/Reference-manual_functions.html#declare_dependency
• https://mesonbuild.com/Wrap-dependency-system-manual.html#wrap-dependency-system-manual

[epsilon-0]

This is interesting, I've never seen this one before. I'll check it out soon.