build-sys: slackware support

build-sys: slackware support

Attached is a patch for slackware support. If you want it some other
way, let me know; I haven't used git before.

Slackware doesn't have gnome or vala, and I'm having some issues with
the vala programs in systemd. I installed the vala compiler, and the
configure script says I have everything required, but compilation
fails (log attached). Is there something I'm missing? I installed
systemd by commenting out the vala programs in Makefile.am.

Malcolm