Autotools updates

Few autotools updates and de-duplication of the README file.
NOTE: not sure about the readme file if that works on windows this way.

Please remember to run "make distcheck" call to test and then release the source tarballs from the tag :)
Looks good. Though there's one reference to DOCBOOK2MAN left that should be cleaned up.
https://github.com/scarabeusiv/OpenXcom/blob/master/configure.ac#L96