Fix `sys/xattr.h` check

I wrote the `configure.ac` wrong, and so we just got no builds
supporting ACLs.

Also, it needs to be more precise because Darwin puts other stuff in
that same header, evidently.

configure.ac

@@ -289,7 +289,8 @@ esac
 AC_SUBST(HAVE_SECCOMP, [$have_seccomp])
 
 # Optional dependencies for better normalizing file system data
-AC_CHECK_HEADERS[sys/xattr.h]
+AC_CHECK_HEADERS([sys/xattr.h])
+AC_CHECK_FUNCS([llistxattr lremovexattr])
 
 # Look for aws-cpp-sdk-s3.
 AC_LANG_PUSH(C++)

src/libstore/posix-fs-canonicalise.cc

@@ -78,7 +78,7 @@ static void canonicalisePathMetaData_(
     if (!(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode)))
         throw Error("file '%1%' has an unsupported type", path);
 
-#ifdef HAVE_SYS_XATTR_H
+#if HAVE_SYS_XATTR_H && HAVE_LLISTXATTR && HAVE_LREMOVEXATTR
     /* Remove extended attributes / ACLs. */
     ssize_t eaSize = llistxattr(path.c_str(), nullptr, 0);