There also some tools I think should be added: I'm not sure if linking to AddressSanitizer is okay (as its (an underused) part of Clang and GCC), but I consider include-what-you-use, unifdef, and maybe C++ Compiler Explorer a worthwhile addition.
What about mbedTLS (formerly PolarSSL)? It's dual licensed though.
Edit: midipix is something one should look out for. What it promises to support and how, sounds much more appealing to me than Cygwin or MinGW-w64, but it's still in early development: http://midipix.org/
Which is not an amazing reason to get into uncanny valley territory with respect to GitHub's design and page layout - to the point of using GitHub's Octicons (which is entirely permitted by the license, for the record, but IMO contributes to an overall uncomfortable feeling when combined with other aspects).