build system housekeeping

[hannesm]

• lwt and cstruct-lwt are an optional dependency again
• use cstruct-lwt in META instead of cstruct.lwt - requires >= cstruct.3.0.0
• build lwt subdirectory only if both lwt and cstruct-lwt are present

[pqwy]

Ah, neat. I was about to complain that the earlier PRs made lwt mandatory.

[hannesm]

a pleasure