Document how to build portable, cross-platform binaries

Creating C++ binaries portable to a given platform (osx, linux, windows) or platform version (ubuntu precise or trusty) is absolutely feasible but a poorly documented topic. Often our goal at Mapbox is to support the latest C++11 and C++14 features but maintain code portable to older operating systems like Windows 7, OS X 10.8, and Ubuntu Precise, or RHEL 6/7. This is also feasible. For example its possible to run C++14 binaries on AWS lambda (built on ubuntu precise) even though you can't easily compile them on AWS Lambda.

We should document how Mapbox writes and distributes portable C?C++ code to make clear when this approach is advantageous and when it is not. And to ensure the techniques are robust and replicable.

This issue is to track:

• getting this all documented here
• Moving the content from https://github.com/mapbox/node-pre-gyp/wiki/External-libraries to this future doc
• Moving binary portability FAQ from https://github.com/mapbox/node-pre-gyp/wiki/FAQ#frequently-ask-questions-about-node-pre-gyp to here

首先查看 node-pre-gyp 的“External libraries” wiki 页面，以及 issue 中链接的二进制可移植性 FAQ。将相关材料整合到计划中的文档中，涵盖可移植的 C/C++ 二进制文件何时具有优势，以及 Mapbox 如何编写和分发这些二进制文件。完成标准是：该主题已在此处记录，并且两个链接的来源都已迁移。