Using a enum is all nice and generic, but at this point it seems unlikely that
we'll add further build modes. But having an enum means that we need to include
the header file with the enumeration whenerever the conditional is used. I want
to use the conditional in log.h, which makes it hard to avoid circular imports.
There is little point in #defining and #undefining CAP_LAST_CAP multiple times.
The check is only done in developer mode. After all, it's not an error to
compile on a newer kernel, and we shouldn't even warn in that case.
