Do not set and empty CMAKE_MODULE_PATH when calling cmake.

It seems like this causes problems on some platforms. If the path is empty we can leave it unset anyway.

This was inspired by http://lists.dune-project.org/pipermail/dune/2016-November/014417.html. Even if that problem might have been caused by something else this MR might have prevented it. It is also a harmless change. Would appreciate somebody from @core merging this.

BTW: How can one summon just core developers/people with merge rights?

Mentioned in commit 68f429a9

Status changed to merged