Warning Suppression and LowLevelSystemSDL Correction

[Saito720]

Organized and fixed issues with the MINGW warning suppression options. I also addressed the return statement in "LowLevelSystemSDL.cpp" for the function "FatalError" that contradicts the function's purpose and threw warnings during compilation.

[jkulawik]

What warnings did FatalError() create? Are you sure that adding an exit there won't interrupt the game where it would work otherwise (even if that's how fatal errors should work... but we need to be sure it's how FG actually used it)? Also is that an hpl function or just smth from std?

As to the CMake stuff:

• why remove -Wno-deprecated-enum-float-conversion?
• please separate (comments and/or separate function call) the first 3 errors from the other ones - this is because the first 3 are false positive or pedantic warnings, and the rest is stuff that should be fixed later.

[Saito720]

Updated "LowLevelSystemSDL.cpp" to have proper indentation and organized "amnesia/src/CMakeLists.txt" warning suppression options.

The reason to keep the change for "FatalError" in "LowLevelSystemSDL.cpp" was explained on Discord in detail. If wanted, the reasoning can be included in the PR as a comment.