Get cross-compilation working again.

[BMagnu]

Fixes some issues in CMake and in the code (notably the case-sensitive includes) that make cross-compilation from linux to windows using MinGW possible again.

To get this to work, you now need the following CMake settings:

#Enable cross-compilation
-DCMAKE_TOOLCHAIN_FILE=./cmake/cross_compile/mingw64.cmake 
#Shaders and default tables are embedded with a custom executable. Since we're building for windows, we're also building this custom embed tool for windows. So just tell CMake to use a different one we've purpose-built for linux in the past. CMake will notify the user if this is not set properly
-DEMBEDFILE_PATH:FILEPATH=<path to a linux FSO build dir>/bin/embedfile
#It's likely possible to get this to build as well somehow, but it's not that critical for distributing test builds, so just disable them for now. CMake will warn the user if these are set differently, but not prevent trying to build with them should a user want to.
-DFSO_BUILD_WITH_DISCORD:BOOL=OFF
-DFSO_BUILD_FRED2:BOOL=OFF
-DFSO_USE_SPEECH:BOOL=OFF
-DFSO_USE_VOICEREC:BOOL=OFF

[Goober5000]

Is it possible to configure CMake to set or prompt for those settings?

One of the things that's nice about the current FSO cmake config is that it does a really good job of figuring almost everything out for you. It would be super helpful if these settings were also automatic, or at least prompted.

[JohnAFernandez]

We could merge this now and make an issue for someone to work on it later.

[Goober5000]

Lafiel said he's already made progress on some of these, so let's hold off just a little bit.

[JohnAFernandez]

Set to draft to help with sorting of open PRs

[BMagnu]

Done the changes, a lot of the required settings are now auto-set and the ones that cannot be are prompted for if they are misconfigured or missing

[Goober5000]

Terrific work! Looks good.