Add missing import/export info for MersenneRNG::unitvector<>
Fix siege-engine to compile on Windows - don't have C++11, and pow() is picky about arg types
Update to Lua 5.2.3
Reapply dfhack-specific lua build configuration.
Rename the LUA_PATH/LUA_CPATH environment variables to avoid conflicts.

If a user has a standalone lua installation with these variables
configured, it should not confuse dfhack into looking for things
in wrong places.
Merge branch 'lua-5.2.3' into develop
Support using multiple lua init scripts per save.

This should make it easier to add and remove init script code by
automated means, or install multiple mods that need such code.