Don't depend on very recent versions of OCaml

Fixes #123

Signed-off-by: David Scott <dave.scott@citrix.com>
Makefile: don't automatically run configure

The user must run
  ./configure
  make

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #124 from djs55/allow-older-ocaml

Don't depend on very recent versions of OCaml
Merge pull request #125 from djs55/allow-older-ocaml

Makefile: don't automatically run configure