OCSIGENGROUP values with spaces breaks sed based makefile rules

[mmorga]

When a group with spaces is used for the OCSIGENGROUP variable, the build fails because the sed based processing of the files like ocsigen_config.ml.in (which end up empty) and result in a compilation failure in baselib.

I found this when attempting to install ocsigenserver 2.7 from opam. My opam global config for group is a name of the form "DOMAIN\Some Group".

I'll submit a PR shortly with a fix.