When cross-compiling, we do not need to add runtime lib dirs.
Also, the cross compilers that are used on OpenWRT do not support
the '-R' option, which causes build failures.

These build failures existed before, but were not noticed,
because it fails only on some setups.
This is because Python's `setup.py` does a lot of voo-doo
automagic that needs handling for some cross-compilation cases.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>