fix for use with git submodules

[FreddyFunk]

This PR solves conflicts when using this repo as a git submodule. This simplifies using this repo within large git projects.

[fogti]

I think the main function should then be made conditional using preprocessor macros.

[FreddyFunk]

Addressed feedback from @zserik .

Both commits might not be the most ideal solution. For the use of this thread pool only the ThreadPool.h file is nessesary while the example.cpp file is intended to be a show case for an example code. Therefore it would be the best solution integrated this example code into the README.md and remove the example.cpp file from the code base completly. Let me know what you think about this suggestion. I can quickly update this pull request to fullfil this suggestion if it gets approved.

[fogti]

Ok, the solution to delete the example.cxx and put the content into README sounds good, especially because the example.cxx is so small.

[FreddyFunk]

Done

[fogti]

ok