bsdgrep: don't link against libregex for bootstrap

r368355 removed the GNU_GREP_COMPAT knob (off by default) and forgot that
bsdgrep may be built/used for bootstrap on some systems.

All base uses should strive to use only POSIX-compliant expressions anyways
and we haven't had libregex by default here up to this point, so just don't
do that if we're bootstrapping.

Note that the resulting binary has the wrong grep -V information as it
falsely claims to be GNU compatible, but it is only for bootstrap.

Reported by: GitHub cross-builds via yuripv