Make last two locations for data available even if XDG_DATA_DIRS is defined in system.
This can be required for non-standart datadir locations, defined at compiletime.
Added compile-time option DATADIR for CMake.
This option can define custom place for datafiles via -DDATADIR.
If this option left undefined, data will be installed to "share/openxcom"
dir (expanded to /usr/share/openxcom).
Same goal can be achieved now with autotools by executing
"./configure --datadir=/usr/share/openxcom"
Merge pull request #474 from winterheart/default-data-dir

Compile-time definition of place datadir