Support the ability to run pip-compile specifying the OS / architecture that should be used for resolving dependencies. Currently it uses the OS where the pip-compile is run. This causes issues such as #333. It also means that if a package does not exist on the current OS (eg for tensorflow-gpu on MacOS`), then compile fails.

Environment Versions

1. OS Type
   MacOS
2. Python version: $ python -V
Python 3.5.3
3. pip version: $ pip --version
pip 9.0.1
4. pip-tools version: $ pip-compile --version
pip-compile, version 1.9.0

Steps to replicate

1. Add tensorflow-gpu>=1.2 to requirements.in
2. pip-compile

Expected result

requirements.txt file with pinned deps. (assuming that --arch manylinux1_x86_64 was set)

Actual result

Could not find a version that matches tensorflow-gpu>=1.2
Tried: 0.12.0rc1, 0.12.0, 0.12.1, 1.0.0, 1.0.1, 1.1.0rc1, 1.1.0rc2, 1.1.0