configure: Fix build for some versions of glibc (9pfs)

Some versions declare open_by_handle_at, but don't define AT_EMPTY_PATH.
Extend the check in configure to test both preconditions.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>

configure

@@ -2812,7 +2812,11 @@ fi
 open_by_hande_at=no
 cat > $TMPC << EOF
 #include <fcntl.h>
+#if !defined(AT_EMPTY_PATH)
+# error missing definition
+#else
 int main(void) { struct file_handle fh; return open_by_handle_at(0, &fh, 0); }
+#endif
 EOF
 if compile_prog "" "" ; then
     open_by_handle_at=yes