distutils uses os.uname() machine for in the build_lib path
https://github.com/python/cpython/blob/2.7/Lib/distutils/util.py#L65
https://github.com/python/cpython/blob/2.7/Lib/distutils/command/build.py#L82
Proposed commit log message:
```
net/py-libnet: Fix build for the platforms that ${MACHINE} != ${ARCH}
Reviewed_by: koobs, sbz
Approved by: koobs (mentor), sbz (maintainner)
Differential_Revision: D15771
```