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 :)
Better test this, as docbook-to-man has caused issues before. See:
http://openxcom.org/bugs/openxcom/issues/645
https://github.com/SupSuper/OpenXcom/pull/839/files