[c++] CMake improvements

[DraTeots]

Various fixes and improvements to CMakeList.txt

• Crete evioConfig.cmake during install. This allows users simply use find_package(evio) in their CMake setups
• Improve Disruptor.cpp handling:
  • better Disruptor search, now one can also provide -DDISRUPTOR_CPP_DIR or to be compatible with environement variable name -DDISRUPTOR_CPP_HOME cmake flags.
  • automatic fetch if external Disruptor-cpp is not found, it is automatically fetched and built by cmake. It can be controlled with DISRUPTOR_ALLOW_FETCH cmake option
• improve lz4 handling (fix false positive error messages and reorder lookup)
• make coherent USE_FILESYSTEMLIB switch between C++17 and boost

Comments:

• I was using latest ubuntu image and the code didn't compiled because of #ifdef __cpp_lib_filesystem. The #ifdef __cpp_lib_filesystem use were mixed with #ifdef USE_FILESYSTEMLIB definition use across files. I changed everything to #ifdef USE_FILESYSTEMLIB, the rationale is that it allows more control over compilation and testing. One can turn it on or off and see the outcome, while __cpp_lib_filesystem work automatically and can produce unwanted untested artifacts like on my setup.

• Added both DISRUPTOR_CPP_HOME and DISRUPTOR_CPP_DIR variables that can be set with -D flag. The rationale: DISRUPTOR_CPP_HOME corresponds to possible environment variable DISRUPTOR_CPP_HOME which I guess is commonly used in coda setup. DISRUPTOR_CPP_DIR is used as in modern CMake <PACKAGE-NAME>_DIR flags are anticipated for setting package directories

• The handling logic for Disruptor-cpp became more complex and occupied a significant portion of the main CMake file. To improve maintainability, I moved it to a dedicated FindDisruptor.cmake script, as is commonly done in such cases.

• New cmake options and flags:

  • option USE_FILESYSTEMLIB - default OFF. When ON - use C++17 instead of Boost
  • option DISRUPTOR_ALLOW_FETCH - default ON. When ON - if disruptor-cpp is not found by EVN or flags, cmake fetches it
  • DISRUPTOR_CPP_DIR and DISRUPTOR_CPP_HOME - possible flags where to look installed disruptor-cpp files

• fixes CMake looks for Disruptor.h in the wrong directories #27

P.S. I carefully tried to keep full backward compatibility. With all changes to cmake file make sure, that with all that changes it behaves as before on systems that uses cmake / environment variables as before. But I don't have coda or whatever setup and can't test changes in production like environments.

[jonzarling]

Thanks! I tested it both ways, building Disruptor-CPP externally and with the -DDISRUPTOR_ALLOW_FETCH=1 option. Both ways of building worked properly for me. I image @carltimmer might want to have a look as well before merging.