Description of the problem / feature request:

We are using libzmq in our project, I created repository_rule to compile it, because there is some platform-dependent headers generated by autoconf or cmake. To cross-compile it, I should add some flags to "./configure" call to generate headers for target platform. But I didn't find any possibilities to provide information about target platform in repository_rule. I couldn't use "select" in rules invoked from WORKSPACE and repository_ctx contains information about host platform only.

Feature requests: what underlying problem are you trying to solve with this feature?

To cross-compile external library sometimes you need to provide target platform information to configure/code-generation stage

Have you found anything relevant by searching the web?

Nothing