Build on bytecode-only compilers

[aantron]

On 4.05.0+bytecode-only without this patch:

$ opam install seq
# + ocamlopt -c -g -I src -o src/seq.cmx src/seq.ml
# /bin/sh: 1: ocamlopt: not found
# Command exited with code 127.
# Makefile:12: recipe for target 'build' failed
# make: *** [build] Error 10

With this patch:

$ opam install seq
Done.

I installed Lwt on 4.05.0+bytecode-only using the patched seq, and it seems to work fine.

Also confirmed that the patched seq still works on regular switches, again by installing Lwt and running its tests. I also looked in lib/seq/ to check that the cmxa and cmxs files are present. The switch was the regular 4.05.0.

I tried to write the code in such a way that it will not break on Windows. In particular, it doesn't assume a which command, or the name /dev/null. As a result, it prints one line to standard error if ocamlopt is not found. However, that line is probably not shown by most package managers. In particular, it has no effect on the opam output.

[c-cube]

Thanks, that's a very good idea.