Add -thread flag to fix compilation on OCaml 4.12

[dhilst]

I had to add this to get it compiled, I was getting this error

#=== ERROR while compiling coq-io-hello-world.1.2.0 ===========================#
# context     2.0.9 | linux/x86_64 | ocaml-base-compiler.4.12.1 | pinned(git+file:///home/geckos/code/coq-io-hello-world#master#5cf7dc99)
# path        ~/.opam/4.12.1/.opam-switch/build/coq-io-hello-world.1.2.0
# command     ~/.opam/opam-init/hooks/sandbox.sh build make -C extraction
# exit-code   2
# env-file    ~/.opam/log/coq-io-hello-world-209338-6aa130.env
# output-file ~/.opam/log/coq-io-hello-world-209338-6aa130.out
### output ###
# Hint: Recursive traversal of subdirectories was not enabled for this build,
# [...]
#   '_tags' or 'myocamlbuild.ml' file). If you have modules in subdirectories,
#   you should add the option "-r" or create an empty '_tags' file.
#   
#   To enable recursive traversal for some subdirectories only, you can use the
#   following '_tags' file:
#   
#       true: -traverse
#       <dir1> or <dir2>: traverse
#       
# make: *** [Makefile:2: build] Error 10
# make: Leaving directory '/home/geckos/.opam/4.12.1/.opam-switch/build/coq-io-hello-world.1.2.0/extraction'

More precisely, by building inside extraction:

➜  coq-io-hello-world git:(master) ✗ make -C extraction 
make: Entering directory '/home/geckos/code/coq-io-hello-world/extraction'
ocamlbuild main.native -use-ocamlfind -package io-system
+ ocamlfind ocamlopt -linkpkg -package io-system main.cmx -o main.native
ocamlfind: [WARNING] Package `threads': Linking problems may arise because of the missing -thread or -vmthread switch
File "_none_", line 1:
Error: No implementations provided for the following modules:
         Mutex referenced from /home/geckos/.opam/4.12.1/lib/lwt/unix/lwt_unix.cmxa(Lwt_main)
Command exited with code 2.
Hint: Recursive traversal of subdirectories was not enabled for this build,
  as the working directory does not look like an ocamlbuild project (no
  '_tags' or 'myocamlbuild.ml' file). If you have modules in subdirectories,
  you should add the option "-r" or create an empty '_tags' file.
  
  To enable recursive traversal for some subdirectories only, you can use the
  following '_tags' file:
  
      true: -traverse
      <dir1> or <dir2>: traverse
      
Compilation unsuccessful after building 5 targets (4 cached) in 00:00:00.
make: *** [Makefile:2: build] Error 10
make: Leaving directory '/home/geckos/code/coq-io-hello-world/extraction'