Add debug postfix option

[hankarun]

Add debug postfix option to the cmake configuration. User can select debug postfix variable with DEBUG_POSTFIX variable.

[kotopesutility]

According to the first item from https://github.com/fpagliughi/sockpp/blob/master/CONTRIBUTING.md you have to send all PR to the develop branch:

1. New and unstable development is done in the develop branch. Please make all pull requests against the develop branch.

[fpagliughi]

Yes, I do prefer PRs against the develop branch, especially when it has diverged significantly from master. It make merging easier, and ensures that the changes reflect the current development state. Since releases of this library are fairly slow, I like to keep master as stable as possible, and only push to it from develop as the new features are tested and settled down.

But that wouldn't prevent me from merging something small like this.

@hankarun Isn't the 'd' postfix only common on Windows systems? I don't think I've ever seen this done on Linux or other *nix systems like Mac.

And if you were to add that, how would apps that use the target be affected? Would they automatically pick up the postfix version if they were compiled for debug?

Apologies for my ignorance. I don't do much Windows programming, and haven't really seen this used before.