configure: find seccomp using pkg-config

On suse we have the header in a subdir inside /usr/include, so
pkgconfig has to be used to find out proper CFLAGS.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-opensuse: add perl-base to prerequisities

It is needed by insserv-compat.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Move pkg-config init to way earlier

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>