This is required for the OpenPGP feature and bug 1588077

Copies of the following libraries should be checked in to the comm-central tree, into the /third_party/ directory:

• https://github.com/rnpgp/rnp
• https://github.com/randombit/botan/
• https://github.com/json-c/json-c

Zlib is a required dependency for RNP which will not be present
on all operating systems that we support.

Bzip2 is a required dependency for RNP which will not be present
on all supported operating systems.

JSON-C is a dependency of RNP.

Modifications are documented in README.rnp and update-rnp.sh

Pushed by thunderbird@calypsoblue.org:
https://hg.mozilla.org/comm-central/rev/46d32cfff30f
Import zlib sources into comm-central. r=kaie
https://hg.mozilla.org/comm-central/rev/fec353984d25
Import bzip2 sources into comm-central. r=kaie
https://hg.mozilla.org/comm-central/rev/38631616c65c
Import json-c sources into comm-central. r=kaie
https://hg.mozilla.org/comm-central/rev/5bda64d96f5f
Import botan sources into comm-central. r=kaie
https://hg.mozilla.org/comm-central/rev/01043748e1e0
Import RNP sources into comm-central. r=kaie

BTW, zlib is also available in mozilla/modules/zlib.
Does comm-central require an additional zlib source?

The RNP shared library is independent of the rest of Thunderbird (libxul really) and doesn't even link to it, so yes.
Yes it's only 100k added to the binary and 2M to the source but it does count and these little things do add up. --enable-system-zlib is honored when building, and will link librnp to whatever libz.so you like. (Similar functionality is planned for bzip2 and json-c.)