Make it possible to add bindist extras externally.

This is useful for, in this case, automatically including some needed
SDL2 DLLs with the Win-Tiles builds (which closes #6422).

Note that, in the process, I split the 'lua' directory into "parts which
are needed when compiling" (in src/lua) and "parts which are needed at
runtime" (left in ./lua). Also note that the remaining special case
(locale inclusion) got its own special Makefile variable, all so I could
avoid doing `cp -R --parents`.
Fixed a bug in elf sample mod.

Mongroup locations are actually relative to their overmap, so no need to get the overmap coordinates. 5, 5 will always be within the designated overmap.
Cleanup consume_fuel()
Merge additional makefile flexibility.
Merge elf horde bugfix.
Merge fuel handling refactor.
Restore default argument to consume_fuel().