[Nix-dev] Some missing build-env features

[Yury G. Kudryashov]

Hi!

The current buildEnv implementation lacks some useful features. I'd like to 
rewrite it but I need some input before I'll start rewriting.

The features I miss in buildEnv.

1) Currently a runtime dependency must be a build time dependency as well. 
This is bad because there are cycles.

2) A package should be able to tell buildEnv to execute some commands after 
symlinking dirs&files. Example: update-mime-info etc.

3) A package should be able to tell buildEnv to skip some directories. 
Various examples are currently hardcoded into build-env script.

4) A package should be able to tell buildEnv about required environment 
variables. Examples: KDEDIRS, XDG_*, *_PLUGIN_PATH etc.

What other features do you miss in the current buildEnv? I'm going to 
propose a syntax for all above after we'll decide on required features.