clingo version 5.2.0

• switch to MIT license
• improve compatibility with abstract gringo
• switch build system from scons to cmake
• improve windows compatibility
• make tests and examples python 3 compatible
• bison and re2c are no longer required to build source releases
• update to clasp 3.3.0
• the CLINGOPATH environment variable can be set
  to control from where to include files in logic programs
• propagators can add variables while solving now
• refactor interfaces (breaking backward compatibility)
  • there is just one solve function now
  • in the C API do not pass structs by value to callbacks
    because FFIs of some languages do not support this
• fix cleanup function
• numerous other bugfixes not listed here