util-linux: fix scanf fallback detection for uClibc-ng

uClibc-ng pretends to be GNU libc 2.2 and then a fallback
scanf check is tried, so that libmount is disabled
afterwards. Add a fix already suggested upstream.
Add librt dependency required for other apps, too.

Signed-off-by: Waldemar Brodkorb <wbx@uclibc-ng.org>