OSL 1.11.8.0

Release 1.11.8 -- 1 Oct 2020 (compared to 1.11.7)

• Fix broken derivatives of the optional "alpha" return of texture calls
  when the normal color channels return don't have their derivatives used
  but the alpha does. #1258
• Building vs Python: instead of defaulting to searching for python 2.7
  specifically and needing to set PYTHON_VERSION if you want (or have) a
  different one, default to whichever version is found. If multiple versions
  of Python are on the system, you can still use the PYTHON_VERSION cmake
  variable to disambiguate which one you want. #1249
• For the experimental OptiX support, explicitly set the OptiX pipeline
  stack size (fixes some bugs). #1254
• ShadingSystem statistics output (which is printed with testshade --help,
  by the way) now includes information about the SIMD capabilities specified
  at OSL build time, those available at runtime, and the versions of LLVM,
  OIIO, and Imath used. This should help in debugging and issue reporting
  by making it easy to know certain build time choices just by running
  testshade. #1258