Check for that header file to prevent HAVE_SIGNAL_H
from being wrongly undefined when compiling auth_cmd.c
on FreeBSD and possibly other operating systems.
This commit kind of reverses one of the changes suggested by Ruben Kerkhof in his pull request #139. I will solve the problem differently to be consistent with his "moderninazation".
Check for that header file to prevent HAVE_SIGNAL_H from being wrongly undefined when compiling auth_cmd.c on FreeBSD and possibly other operating systems.