Conda-build: [Feature] Add a new command line argument to set environment variables

[frol]

Currently, you can specify a list of passthrough environment variables in meta.yaml (build/script_env). However, there are use-cases when it is not feasible to list all possible env variables. One example of such a useful environment variable is MAKEFLAGS and it is even made into the conda-build core. Imagine you don't need to make extra PRs just to pass through a new build-system specific variable.

Please, consider the following syntax (copied from docker run command):

$ conda build --env "MAKEFLAGS=--jobs 4" --env "CPPFLAGS=-Wall -Werror" ./conda.recipe/

This way we won't worry about implicitly and unexpectedly passed settings from a host system. Also, there would be no need to list all sorts of possibly useful only for build systems env variables in recipes.

P.S. It might be also a good idea to implement a config file for conda-build where these environment variables and other useful settings can be saved and reused.

/cc @jschueller @groutr @msarahan @jakirkham

[msarahan]

Yep, great idea. The limiting factor here has been time. See #966 for progress. This is one step behind #953 on my priority list, but I hope to get to it very soon.

[jakirkham]

Yep, those both sound like great ideas, @frol. I'd much prefer we even specify MAKEFLAGS this way instead of having it setup for auto-passthrough.

[frol]

@jakirkham That was also a part of the intention behind it. Currently, somebody can potentially hit some nasty issues if they use something fancy in their MAKEFLAGS.

[jschueller]

In that case you may want to revert these: https://github.com/conda/conda-build/pull/917 https://github.com/conda/conda-docs/pull/270

cc @jakirkham

[jakirkham]

cc @ocefpaf