libxfs: port IS_ENABLED from the kernel

libxfs: port IS_ENABLED from the kernel

Port the IS_ENABLED macro from the kernel so that it can be used in
libxfs.  This requires a bit of hygiene on our part -- any CONFIG_XFS_*
define in userspace that have counterparts in the kernel must be defined
to 1 (and not simply define'd) so that the macro works, because the
kernel translates CONFIG_FOO=y in .config to #define CONFIG_FOO 1.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>