See src/README.md for the README that ships with snappy.

Mozilla does not modify the actual snappy source with the exception of the
'snappy-stubs-public.h' header and one small patch to resolve implicit
constructor warnings. We have replaced its build system with our own.

Snappy comes from:
  https://github.com/google/snappy

To upgrade to a newer version, run:
  ./mach vendor other-licenses/snappy/moz.yaml

This fetches the tagged release, applies 01-explicit.patch and runs
update-version.py, which stamps the version into 'snappy-stubs-public.h' and
the SNAPPY_VERSION static_asserts in dom/. After updating, compare
'snappy-stubs-public.h' against the new 'src/snappy-stubs-public.h.in' for any
other changes. Note that we don't bother trying to detect the availability of
sys/uio.h and unconditionally define the iovec type instead for all platforms.