1.5.3

Added:

• New CYCLUS_PLATFORM string for uniquely identifying the
  platform. This is initialized with the cyclus_platform()
  macro.
• New download_platform() macro for downloading a tar.gz
  file and its platform-specific equivalent.

Changed:

• Cyclus agent libraries will now have the the install directroy and
  rpath correctly set to when using the install_agent_lib() in CMake.
• Updated how fast compiling downloads and searches for files so that
  only platform-specific files are grabbed.

Deprecated:

• CMake policies CMP0040 and CMP0042 supressed.

Fixed:

• Fixed issue with cyclus_unit_tests not being able to find
  the agents library in linking on macOS.
• Fixed SQLite backend bug with Boost v1.66.
• CMake RPATH now correctly uses semicolon delimiters, rather
  than colons.
• DynamicLoadingTests now properly start/stop Python.