Use tmp-PKG-TARGET as temporary dir

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Always symlink to log/PKG_TARGET

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Fix libxvidcore shared DLL location

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Merge pull request #348 from TimothyGu/parallel

Allow building packages for different targets simultaneously using multiple `make` instances