Reorganize build scripts for libuv.

- Create separate libs for each architecture. OS X doesn't need this
  (we just build a universal binary), but it will help Linux.
- Move the libuv build stuff into wren.mk where the actual dependency
  on the lib is.
- Download libuv to deps/ instead of build/. That way "make clean"
  doesn't blow it away.
- Don't redownload libuv unless needed.

.gitignore

@@ -5,6 +5,7 @@ wren
 
 # Intermediate files.
 build/
+deps/
 .sass-cache/
 
 # I leave a temporary Wren script at the top level so that I can quickly test