When searching for options like -n in MAKEFLAGS, current code may result
in a false positive match when make is invoked with long options like
--no-print-directory. This has been observed with certain versions of
host make (e.g. 3.82) while building the Qemu package in buildroot.

Filter out such long options before searching for one-character options.

Signed-off-by: Alexey Neyman <stilor@att.net>
Message-Id: <20210722020846.3678817-1-stilor@att.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>