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`.