Allow setting of config file path without requiring environment variables.

[zeyus]

On non-desktop platforms, often apps are sandboxed and don't have (read/write) access at all to $HOME, /etc/ or even the current working directory. These paths may not exist.

In addition, environment variables don't necessarily make a lot of sense in the context of a mobile app, and the application data paths might contain GUIDs or other aspects of the path that make it difficult to determine at compile-time.

This PR adds the ability to set a configuration file path for liblsl provided that the static member function lsl_set_config_filename is called before any other LSL function.

In addition, there's also an option to read config directly from a string stream via set_api_config_content which is loaded into the INI and discarded.

[zeyus]

@cboulay is there something I should do differently for this PR? Or is it not functionality that you would be interested in adding?

[cboulay]

Hi @zeyus
Sorry, this one got auto-closed when I deleted a branch and I failed to retarget it. I'll try to fix that now.