Add ifdef to disable pthreads usage

[Sokolmish]

Hello. I used nanoflann in my project and got a linker error because of pthreads usage (std::async). Since I don't use parallel construction and don't want to link my code with pthreads, I've just removed these calls from the library. I think that it might be convenient for others to do the same by just adding a define before including the header, so I made this PR.

I was thinking about adding the possibility to disable multi-threading to CMake, but I'm not an expert on it. If you think that it is necessary, I'll figure it out and add it to the PR.

Also, I've included in the readme all the ifdefs I found in the header.

[jlblancoc]

LGTM, thanks! I think it's not worth changing cmake for this, since interested users may just define the preprocessor symbol from their side if needed.