the windows builder has 2 different build folders for the debug and
release libs, hardcoding the path like this leads to an extra 1GB
of downloaded files. The windows builder supplies a single PACKAGE_DIR
for both but gets ignored.
This change promotes PACKAGE_DIR to be cached in CMakeCache so it can
be changed if needed.
Differential Revision: https://developer.blender.org/D16213
Reviewed by; brecht, dfelinto
14c218b3fd