add cmake check for libattr

[adammoody]

This adds a cmake find_package check for libattr, which is used for attr_copy_action when copying extended attributes. When xattr support is enabled, libattr is marked as required. However, this also adds compile-time checks in the code to disable libattr code if is was not found so that it could be optional rather than required.

[adammoody]

@ofaaland , would you please review this when you get a chance? If you have suggestions, please let me know.

In case someone doesn't have libattr installed, I protected the libattr-related lines with a HAVE_LIBATTR check. For now, I've left the cmake check as required, but we could likely back that off to make it optional.

I also dropped the input ctx variable and replaced it with NULL since we weren't using it anyway. To use, I think we'd want to fill in the fields of the struct with pointers to functions, as in the example here:

https://github.com/philips/attr/blob/1cc88bd4c17ef99ace22c8be362d513f155b1387/examples/copyattr.c#L89

[ofaaland]

LGTM
I think we should leave LibAttr as "REQUIRED" for the moment since there is no easy way for the user to tell whether libattr support was built (if they don't know about tools like ldd)

[daltonbohning]

@adammoody To be clear, the current (and intended) behavior is that libattr is required when ENABLE_XATTRS=ON?

[adammoody]

@daltonbohning , yes, that's the goal, but we're assuming that one has libattr available. If we find common setups that use xattrs but do not have libattr installed, I think we could do more work to make libattr optional.

[daltonbohning]

@daltonbohning , yes, that's the goal, but we're assuming that one has libattr available. If we find common setups that use xattrs but do not have libattr installed, I think we could do more work to make libattr optional.

@adammoody Thanks. Luckily, so far it seems our (DAOS) setups do have libattr installed/available.