At Bloomberg, we just published our lowest-level C++ standard library (and plan to add more layers on top).[1] Everything we do is based upon this library and all the higher level layers which are not OSS yet. (i.e., No external 3rd party libs in any of these lowest layers) We CC-BY-SA'd our code standards as well if you're interested.[2]
We went with gyp because we wanted to natively support MSVC and XCode projects as well as standard GNU make environments. Our production build system actually has simple text metadata files that live along-side the code and it generates all the required files as well. We felt that it easier to provide a gyp based system instead of also open-sourcing the entire Perl-based build system which was tied to our custom data format and not generally useful to anyone else. We generate the gyp input files from the metadata on our end and then anyone using the open-source project can use gyp to do whatever they need.
If we get to the point where OS distros want to package the libs, we may opt to generate autotools files, but that was too much work to be a roadblock from open-sourcing the lib.
[1]: https://github.com/bloomberg/bsl [2]: https://github.com/bloomberg/bsl/wiki/bdestds.pdf