Fix Linux compilation for x86 (32-bits)

[AldoMX]

Historical Context

When I started StepMania AMX more than 15 years ago, I didn't know how code or use source control. This repo is an attempt to reconstruct the timeline, so I will be rewriting history and force pushing changes.

The last time I attempted to compile the game in Linux it worked fine, but the videos were not working. At that point in time I thought that the code was irreparable and gave up any effort to fix it. Nowadays I know that the issue is that the Linux source code attempts to read the system ffmpeg instead of the one provided by the source, and ffmpeg is a particular headache.

What needs to be done?

Review each one of the ~ 600 commits, and update the linux build files to apply changes in the exact point in time where they were introduced.

Update the commits where I updated dependencies to fix Linux compilation. That's like cheating because I'm changing the past and erasing the fact that I gave up Linux support, but I don't want to leave the project unbuildable when I know that the build issues were minor.

NOTE: Some features, such as the update prompts or the bigger splash screen should be wrapped in #if defined(WIN32) compiler directives as dealing with GTK versions is a huge effort and I prefer to reimplement them in the latest version that this project will depend on.