libxfs: add autoconf mechanism to override system header fsxattr

libxfs: add autoconf mechanism to override system header fsxattr

By default, libxfs will use the kernel/system headers to define struct
fsxattr.  Unfortunately, this creates a problem for developers who are
writing new features but building xfsprogs on a stable system, because
the stable kernel's headers don't reflect the new feature.  In this
case, we want to be able to use the internal fsxattr definition while
the kernel headers catch up, so provide some configure magic to allow
further patches to force the use of the internal definition.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
v2: Remove the user-visible configure option but leave the fsxattr
override ability so that subsequent patches can trigger it if
necessary.