Need way to specify target platform

[excavador]

Hi!

I am using your rules, and rules are awesome.
I have single small problem: on OSX I am building PEX binary and then put it to centos-based docker image.

As result, PEX image does not work because it includes OSX-based numpy/other libraries with C modules.

It would be great to provide some way to build PEX-binaries for target platform (ideally - handle "--cpu" flag from bazel, which used for cross-compile)

Thank you!

[benley]

Cross-compilation support would be very nice to have, indeed! I'm not certain but I think it would be difficult to implement today, because of the current lack of python support in bazel toolchains. The py_runtime rule type helps somewhat, but I'm not sure how to go about specifying that a build should use different Python versions for build steps (on the local system) and the target system (cross compiling target for C modules). Over in rules_python there are some related topics, like bazelbuild/rules_python#42.

[benley]

Also somewhat related:

bazelbuild/rules_python#48
google/subpar#55
bazelbuild/bazel#1475