Bump GMT to 6.4.0

[maxrjones]

No description provided.

[seisman]

Also need to update the GMT version information in README.

[maxrjones]

Not sure if I should ignore the validator failure for the Julia components

[PaulWessel]

Not sure if I should ignore the validator failure for the Julia components

Not sure what that means - probably @joa-quim territory?

[joa-quim]

Not our fault and really nothing we (I) can do for now.
The problem is that Unix once again played its favorite game: breaking dependencies.

On *nix, when no GMT if found, GMT.jl automatically installs one via Conda. But Conda decided to start use the shiny shiny new GCC12 that has a new libstdc++ incompatible with predecessors. So those GMT builds cannot be used with current Julia(s) because of that. Add to it that they (the master Julians) have not been able to cross compile libraries from GCC 12.

See what Windows users are safe from because we use the obvious MSVC framework to build our binaries?

Failed to precompile GMT [5752ebe1-31b9-557e-87aa-f909b540aa54] to /srv/julia/pkg/compiled/v1.7/GMT/jl_gIdOKB.
ERROR: LoadError: could not load library "/srv/conda/envs/notebook/lib/libgmt.so"
/srv/julia/bin/../lib/julia/libstdc++.so.6: version `GLIBCXX_3.4.30' not found (required by /srv/conda/envs/notebook/lib/./libgdal.so.31)

[maxrjones]

Thanks for the explanation. Merging since the problem exists for both 6.3 and 6.4 and as Joaquim said, there's nothing we can do.