build-sys: check scanf %ms modifier

build-sys: check scanf %ms modifier

Without the check libmount builds on systems that has older than 2.7
glibc are silently unsuccessful.  The missing %ms modifier will, at
least, result on such system missing output of findmnt and lsblk
commands.  If either %ms or %as modifiers are present the libmount
build is disabled.

Based on patch from: Sami Kerola <kerolasa@iki.fi>

Signed-off-by: Karel Zak <kzak@redhat.com>