Make configure builtin-aware.

This patch adds a new option to configure: --enable-linux-builtin. When
this option is used, the following happens:

• Compilation of kernel modules is disabled.
• A failure to find UTS_RELEASE is followed by a suggestion to run "make prepare" on the kernel source tree.

This patch also adds a new test which tries to compile an empty module
as a basic toolchain sanity test. If it fails and the option was
specified, the error is followed by a suggestion to run "make scripts"
on the kernel source tree.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/zfs#851