Twice now I've been bitten by building agaist a kernel which is
configured such that it is incompatible with the CDDL license. These
build failures don't occur until the linking phase at which point they
simply callout the offending symbol. No location information can be
provided at this point so it often can be confusing what the problem is
particularly when building against a new kernel for the first time.
To help address this I've added a configure check which can be extended
over time to detect known kernel config options which if set will break
the ZFS build. Currently I have just added CONFIG_DEBUG_LOCK_ALLOC which
makes mutex's GPL-only and is on by default in the RHEL6 alpha builds.
I know for a fact there are other similiar options which can be added
as they are encountered.
fd7578215e