Fix build on ocaml 4.06

Ocaml swiched to immutable strings by default in this version. Cool should
probably be ported to use those, but in the meantime we just disable the feature
to make it build on newer ocaml versions.

_oasis

@@ -27,8 +27,8 @@ Library libcool
 
   # Warning: we use C++ sources in .c files because oasis does not recognize
   # cpp-files
-  NativeOpt:        -cc g++
-  ByteOpt:        -cc g++
+  NativeOpt:        -cc g++ -unsafe-string
+  ByteOpt:        -cc g++ -unsafe-string
   CCOpt: -std=c++98 -x c++
   # glpk_stub is not included here, because it's not being used by
   # any code right now. Add it back if you need it.