BuildCRX is a standalone binary for Windows/Linux to RSA sign and pack a zip-file containing chrome extension data into a CRX file.
Chrome extensions are contained in a .CRX package, which consists of the following elements -
The chrome and chromium binaries can do this for you, however if you are using a build-system, you probably don't want to install chrome or chromium. There are several utilities to do the same thing without using chrome, which use a combination of a scripting or shell technology, and OpenSSL.
Unfortunately those tools require an interpreter or shell to run - which could pose an issue if you are attempting to build your CRX within windows - enter BuildCRX!
BuildCRX is a windows/linux utility written in C, which has no external dependencies; it does not require chrome/ium or the OpenSSL binaries. BuildCRX statically includes libcrypto from the OpenSSL project, so it should run without installing any other libraries.
This utility has little purpose on linux in my opinion, since you can use any number of the alternative methods - however if you don't want to setup OpenSSL in your Linux build environment, you could use this utility as well - as it is cross platform.
I have created a list of alternative utilities for building .CRX packages here