See src/README 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. We have replaced its build system with our own.

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

We are currently using revision: 1.1.9

To upgrade to a newer version:
  1. Check out the new code using subversion.
  2. Update 'snappy-stubs-public.h' in this directory with any changes that were
     made to 'snappy-stubs-public.h.in' in the new source.
  3. Copy the major/minor/patch versions from 'CMakeLists.txt' into
     'snappy-stubs-public.h'.
  4. Copy all source files from the new version into the src subdirectory. The
     following are not needed:
       - 'CMakeLists.txt', 'snappy_benchmark.cc', 'snappy_test_data.cc',
         'snappy_test_data.h', and 'snappy_test_tool.cc' files
       - 'cmake' subdirectory
       - 'docs' subdirectory
       - 'testdata' subdirectory
       - 'third_party' subdirectory
  5. Update the revision stamp in this file.