configurator: C_define: When the number is ≥ 0, prefix it with '0'
configurator: C_define: When the number is ≥ 0, prefix it with '0'

The function int_of_string in OCaml 4.02 does not support a leading '+'.

Fixes https://github.com/ocaml/dune/issues/1738

Signed-off-by: Christophe Troestler <31b857c84ef8b5a36e0408d32d21a2c72c6b4dca@umons.ac.be>

janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder