Documentation fails to build on docs.rs

[cessen]

The docs.rs servers don't have any version of the C++ OpenEXR library installed, so building openexr-rs always fails, preventing our docs from being published there.

I recently looked into that, and according to the documentation on their site they provide an environment variable, DOCS_RS, that you can use in build scripts for situations just like this. In our case, we can use it to conditionally disable all of our C/C++ build steps in openexr-sys, which allows openexr-rs to "succeed" at building without the C++ OpenEXR library present, even though it would fail if you actually tried to link it into anything.

I've made precisely that change in commit dba20d8, which should allow documentation generation to succeed on docs.rs from now on. However, I don't want to cut a new release just for documentation generation, so I'm opening this issue so that there's visibility on this situation until the next release.