spec: fix broken reliance on systemd_requires defined in SRPM build time

Actually orthogonal to how buildroot (usually used also for building
source packages alone) looks like/which packages are always present.
In a container, for instance, may not be any systemd (and hence its
RPM macros) at all.

Still rather fragile but working "is systemd a native platform where
you build this SRPM" detecting macro borrowed from upstream pacemaker
spec (I happen to author there, for completeness and as a reminder,
please keep these snippets in sync if at all possible).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>