Comment 1 for bug 601342

This problem is caused by an inconsistency in the linux-headers packages.
Some of them provide the symlink, some other don't.

The latest oss4-dkms package (4.2build3002-1) now explicitely requires a linux-header package to be installed, which should fix the problem is most cases.

In general, I am not sure that pointing to /usr/src/... is a good solution, the existence of a /lib/modules/`uname -r`/build symlink is a pretty standard requirement for generic module build so this problem will happen again for other external modules and I would recommend investigating why this symlink does not exist in some installs...