Update compiling_with_gnu_make.md

It looks like Markus Mottl has moved away from bitbucket. The [old link](https://bitbucket.org/mmottl/ocaml-makefile) goes to a redirection page that suggest [this page](http://mmottl.github.io/ocaml-makefile/) I used in this pull-request. One might prefer to directly link to the source repository, but this page's documentation is interesting and seems fairly complete -- one may wonder whether it could even replace the current `compiling_with_gnu_make` page.