Additional fixes for system w/o systemd

Additional fixes for system w/o systemd

For systems without systemd, some variables may get expanded into empty
strings, eg.:

if [ -z "$(SYSTEMD_UNIT_DIR)" -o ! -d $(DESTDIR)$(SYSTEMD_UNIT_DIR) ]...

may be expanded into:

if [ -z "" -o ! -d  ]...

when DESTDIR is also unset. This causes an error when executed:

/bin/sh: line 0: [: argument expected

The solution is to enclose these variables in double quotes.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>