Compile-time definition of place datadir

Some Linux distribution have own rules about installing game assets. For example, Gentoo rules states that executables should be placed into /usr/games/bin and data - into /usr/share/games.
This PR enables new compile-time option - DATADIR, with can be defined:
- in cmake as cmake \<dir_with_openxcom_sources\> -DDATADIR="/own/path/to/data";
- in autotools as ./configure --datadir="/own/path/to/data".

If this option remain undefined data-files will be installed in /usr/share/openxcom.