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 
#FSO needs access to some of the dll's, so we need to provide it any reasonably up-to-date build directory from windows to pull them from
-DIMPORT_EXE_DIR:FILEPATH=<path to a windows FSO build dir>
#Freetype, for some reason, needs additional special handling, and a direct path to a windows freetype281.lib, typically available here
-Dfreetype_LOCATION:FILEPATH=<path to a windows FSO build dir>/lib/prebuilt/freetype/lib/freetype281.lib
#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.
-DEMBEDFILE_PATH:FILEPATH=<path to a linux FSO build dir>/bin/embedfile
#SIMD detection does not work since this relies on making and running a custom executable as well, so just force SSE2
-DFORCED_NATIVE_SIMD_INSTRUCTIONS:BOOL=OFF
-DFSO_INSTRUCTION_SET=SSE2
#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.
-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