Add Support for DLL Exportable Functions

[yesilcimenahmet]

Thank you for your excellent work on fpng. I have been using the library in my project and found it to be extremely efficient. However, I have a specific use case where I need to compile fpng as a DLL (shared library) with some functions exported for use in external applications (e.g., Delphi).

Currently, while attempting to adapt the library for this purpose, I noticed that the library doesn't provide a straightforward way to export functions like fpng_encode_image_to_memory or similar. This makes it challenging to dynamically link the library in projects where a DLL-based architecture is preferred.

[lvandeve]

This is actually the lodepng github repo instead of the fpng one, probably you meant to open it there :)