Support a system-wide edflib (fix #149)

[musicinmybrain]

Adds an environment variable SYSTEM_EDFLIB. When it is set empty or to 1, the bundled EDFlib sources are not compiled, and a system-wide edf library is linked instead.

[skjerns]

Thanks for the PR, that looks interesting indeed. I'll have a look at it the coming days. As it is purely additional and does not infer with the commonly used install mode, it should be fine to merge.

[musicinmybrain]

I see now that the bundled EDFlib is actually a fork, with at least some added error codes and an extra size-check parameter to edfopen_file_readonly. As long as pyedflib needs to fork EDFlib, this PR isn’t actually useful.

[skjerns]

I see now that the bundled EDFlib is actually a fork, with at least some added error codes and an extra size-check parameter to edfopen_file_readonly. As long as pyedflib needs to fork EDFlib, this PR isn’t actually useful.

Yes, that is true. However, we could think about making some of the changes optional, I don't think all of them are extremely important. On the other side, it should be no problem to recompile the source for installing the lib.