[C++] Use vendored flatbuffers in Meson configuration

[WillAyd]

Describe the enhancement requested

While submitting Arrow to Meson's wrapdb, the fact that Arrow requires a very specific version of flatbuffers is problematic when using it amongst other subprojects, which may or may not require the same version (see discussion at mesonbuild/wrapdb#2099 (comment))

I have explored making it so that CMake would allow a dynamic version of flatbuffers in #46471 , but that request is more complex than it seems, as flatbuffers creates not just a library but an executable for compilation. I think the only way to do that with CMake would be to use a rather complicated ExternalProject_Add configuration (FetchContent_MakeAvailable does not work, as it does not expose the flatc executable at config time for the rest of the CMake configuration).

To avoid that CMake complexity for now, it's probably easier just to remove the Meson flatbuffers subproject usage and instead use the internally vendored copy

Component(s)

C++

[kou]

Issue resolved by pull request 46484
#46484