deps: upgrade embedded zlib to version 1.2.11
The current version of zlib bundled with libgit2 is version 1.2.8. This
version has several CVEs assigned:
- CVE-2016-9843
- CVE-2016-9841
- CVE-2016-9842
- CVE-2016-9840
Upgrade the bundled version to the current release 1.2.11, which has
these vulnerabilities fixes.
Add ZLib as a built-in dependency
I don't know if this is good or bad. This lets libgit2 compile cleanly
on any platforms without any external dependencies, but adds a little
bit of bloat...
Let's test this out and see what happens.