fix sed in Makefile when path contains '+'

to avoid this error:

```
sed -e "s+__OCAMLC__+/home/jdimino/.opam/4.01.0+fp/bin/ocamlc+" < scripts/ocamlc > /home/jdimino/code/opam-tmp/system/bin/opam-doc-ocamlc
sed: -e expression #1, char 41: unknown option to `s'
make: *** [install] Error 1
```