- do not link with -llua. We should not link apk or lua module with it.
- default to lua 5.2
- allow override the lua version with LUA_VERSION
- allow override the lua pkg-config package with LUA_PC
- only add the pkg-config lua --cflags to the lua module

 * basic code for a backtracking, forward checking dependency satisfier
 * works better when there are tricky dependencies to solve
   (when can't just upgrade everything to most preferred versions)
 * the new code always evaluates all of 'world' constraints
   (old code just does incremental updates based on heuristics)
 * is probably somewhat slower than old code (probably unnoticeable
   difference in most cases)
 * makes easier to write support for provides and repository pinning
 * test applet and a bunch of test cases added which uses the new code
 * from the old feature set install_if is not yet implemented

This allows users to have a collection of build variables set without
needing to specify them at build time every time.