Revise the build system with a Makefile.

[Amavect]

The current build system is adequately documented, but cumbersome to set up and relies on a myriad of scripts that are shoved away in the build folder. I have been working on writing a Makefile to ease cross-platform building and packaging, and providing better build documentation.

• All documentation relating to the build is within README.md instead of shoved away in the wiki. The wiki pages should be removed.
• Instead of directly installing MinGW-w64 on Windows, MSYS2 will be used for building on Windows, simplifying OpenAL installation.
• System-specific configuration files are used, resulting in the CONF=os make pattern. This includes the variables defined in the corresponding Make.$CONF file. This leaves cross-compiling from other OSes a possibility, though Linux is currently only supported.
• Go-openal is modified to static-link the Windows executables. This allows us to package all binaries in a single zip file and not worry about distributing DLLs. The OpenAL license needs to be distributed, as a result.
• CONF=linux make pkg builds and packages in a single step, which makes a binaries-only zip and an Elecbyte-screenpack zip.
• Docker is not necessary for any steps.

The fork is here: https://github.com/Amavect/Ikemen-GO

I still have a few finishing touches to make relating to cross-platform build documentation, license documentation, naming the default target, and windres icon files. Then, full-release testing can begin.

[Windblade-GR01]

Docker is currently used for the CI/CD system. So these could not be removed.

But this is a big improvement over the current .sh based system. I'll merge it.

And, thanks for of making these. 😄