build: fix build failure due to missing libxattr

build: fix build failure due to missing libxattr

Configure is supposed to detect insufficient XATTR support.
However, if a system has the required headers, but no library,
the configure script would mistakenly enable USE_XATTR.
* m4/xattr.m4 (gl_FUNC_XATTR): If the attr_copy_file function
is not found, don't set USE_XATTR.
Nelson Beebe reported a link failure on RHEL 5.3.
Also, do not let the combination of --disable-xattr and
a stray LIB_XATTR environment setting perturb the build.
* NEWS (Build-related): Mention it.