meson: provide fallback for *proto dependencies

Meson has a built-in facility to use bundled versions of dependencies
if system packages are too old. Enable for xorgproto after 8e504d8b:

Run-time dependency xproto found: YES 7.0.33
Run-time dependency randrproto found: YES 1.6.0
Run-time dependency renderproto found: YES 0.11.1
Run-time dependency xextproto found: YES 7.3.0
Dependency inputproto found: NO found 2.3.2 but need: '>='
Found CMake: /usr/local/bin/cmake (3.20.2)
Run-time dependency inputproto found: NO (tried pkgconfig and cmake)
Looking for a fallback subproject for the dependency inputproto ERROR: Neither a subproject directory nor a xorgproto.wrap file was found.
5 jobs for meson-proto in 6 minutes and 3 seconds (queued for 1 second)