Fedora 28: Fix "Macro %_dracutdir has empty body"

If you run ./configure --with-config=srpm, it will not trigger
the user m4 scripts to populate the dracut and udev directories.
This causes a build error on Fedora 28. Make the dracut and
udev lines conditional to get around this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7326
Closes #7328