hack to compile with my debian jessie using a recent ocamlgraph package


* building( make all ) in git cloned did 

 [!] Git repo info for opalang not found
\+ /usr/bin/ocamlc -c -g -warn-error A -w L -w Z -I compiler/buildinfos -I ocamllib -I tools -I lib -I compiler -I tools/build -o compiler/buildinfos/buildInfos.cmo compiler/buildinfos/buildInfos.ml
File "compiler/buildinfos/buildInfos.ml", line 20, characters 2-17:
Error: Unbound value opa_git_version

=> I added opa_git_version additionally to ${repo}_git_version to be able to compile.

* after first fix another build error popped up 

\+ /usr/bin/ocamlc -c -g -thread -warn-error A -w L -w Z -I /usr/lib/ocaml/zip -I /usr/lib/ocaml/ocamlgraph -I /usr/lib/ocaml/ulex -I ocamllib/libbase -I tools -I ocamllib -I lib -I compiler -I tools/build -o tools/odep.cmo tools/odep.ml
File "tools/odep.ml", line 103, characters 35-122:
Error: Some record fields are undefined: sg_parent

After review it come form a ocamlgraph api changes 
[ https://github.com/backtracking/ocamlgraph commit e5aa3733de34fc5da4aa3acb82862b3e88a3062a did add sg_parent field, fix code to compile with that change. ]

=> i added sg_parent = None when needed .

( my first pull request so be kind with this post quality ).