Support OCaml 4.06.0

[mato]

Due to the refactoring of the compiler build system in 4.06.0 this is not the usual trivial change, and now needs separate paths for >= 4.06.0. I'm not entirely happy with the hacks involved but "it works for me".

Tested on Linux with ukvm and the mirage-skeleton device-usage/network example; I'd appreciate more testing and review from others before merging this (/cc @hannesm).

[hannesm]

@mato thanks for doing this.

We now need separate paths (for <4.06 and >= 4.06) now, but do we really need the bits from the bigarray subdirectory? ocaml/ocaml#1077 is where most symbols got moved into the OCaml runtime.

The header file dance is due to ocaml/ocaml#1127

I'll test this tomorrow and report back!

[hannesm]

I tested this as is on FreeBSD, and it works nicely!

[mato]

@hannesm Thanks for the pointers to the OCaml issues. Based on my reading of the comments there it looks like my approach is correct. I've pushed a small change that improves the naming for the variable triggering the >=4.06.0 build paths, otherwise I think this is fine.

Regarding the continued need for libotherlibs.a post 4.06.0, I don't know enough about whether or not those symbols are used further up the stack. At this point I'd just merge as-is since it won't hurt, and anyone who wants to investigate further can do that in a followup PR.

[hannesm]

@mato yes, fully agreed. and to me it looks like at some point in 4.07+, OCaml may include the entire bigarray (apart from the map_file) into its stdlib. once this happens, we can get rid of otherlibs, but with 4.06 we need it.