Dynamically set the program version from the Makefile

The program version string used to be a #define. Now we set from the
Makefile using git-describe to get the version string. We now display
something like

    alterMIME v0.3.11-22-ga3cf3bb -
      https://github.com/ac000/altermime/tree/fixs (August-2014).
      alterMIME by Paul L Daniels - http://www.pldaniels.com/altermime

Signed-off-by: Andrew Clayton <andrew@zeta.digital-domain.net>