abc.sh: turn on dotglobbing to ensure archive function captures dotfiles

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>
Allow the OS (freebsd or linux) to be specified as a make option

* OS specific commands (namely `date`) within abc.sh are
  now inserted via the Makefile

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>