OpenImageIO 2.1.10.1

Release 2.1.10.1 (10 Jan 2019)

• Automatically detect the need to link against libatomic (fixes build on
  some less common platforms, should not affect Windows, MacOS, or Linux on
  x86/x86_64 users). #2450 #2455
• Fixes to unordered_map_concurrent.h that affect some users who it for
  things other than OIIO per se (recent changes to the internals broke its
  use for the default underlying std::unordered_map). #2454
• Bump the minimum pybind11 vesion that we auto-download, and also be sure
  to auto-download if pybind11 is found on the system already but is not an
  adequately new version. #2453
• If libsquish is found on the system at build time, use it, rather than
  the "embedded" copy. This can improve build times of OIIO, and also helps
  us comply with Debian packaging rules that forbid using embedded versions
  of other Debian packages that can be used as simple dependencies. #2451
• Fixes to formatting of man page generation (resolves warnings on Debian
  build process).