scripts/mkcompile_h: don't test for hardcoded paths

Don't test for /bin/{dnsdomainname,domainname}, simply try to execute
the command and check if it returned something.

Reported-by: Glenn Sommer <glemsom@gmail.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Tested-by: Glenn Sommer <glemsom@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>