configure.ac: check for xattr support both in libc and in -lattr

In 2015, the attr/xattr.h header was fully removed from upstream attr.

In 2020, rsync started preferring the standard header, if it exists:
#22

But the fix was incomplete. We still looked for the getxattr function in
-lattr, and used it if -lattr exists. This was the case even if the
system libc was sufficient to provide the needed functions. Result:
overlinking to -lattr, if it happened to be installed for any other
reason.

```
checking whether to support extended attributes... Using Linux xattrs
checking for getxattr in -lattr... yes
```

Instead, use a different autoconf macro that first checks if the
function is available for use without any libraries (e.g. it is in
libc).

Result:

```
checking whether to support extended attributes... Using Linux xattrs
checking for library containing getxattr... none required
```

Signed-off-by: Eli Schwartz <[email protected]>

Author: eli-schwartz

configure.ac

@@ -1392,7 +1392,7 @@ else
 	AC_DEFINE(HAVE_LINUX_XATTRS, 1, [True if you have Linux xattrs (or equivalent)])
 	AC_DEFINE(SUPPORT_XATTRS, 1)
 	AC_DEFINE(NO_SYMLINK_USER_XATTRS, 1, [True if symlinks do not support user xattrs])
-	AC_CHECK_LIB(attr,getxattr)
+	AC_SEARCH_LIBS(getxattr,attr)
 	;;
     darwin*)
 	AC_MSG_RESULT(Using OS X xattrs)