[cmake, docs] msys: allow bundled FFmpeg, update dep/caveat docs, faster linking on Release mode (#2921)

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/2921
Reviewed-by: Caio Oliveira <caiooliveirafarias0@gmail.com>
Reviewed-by: Lizzie <lizzie@eden-emu.dev>

docs/Caveats.md

@@ -13,12 +13,11 @@
 
 ## Arch Linux
 
-- httplib AUR package is broken. Set `httplib_FORCE_BUNDLED=ON` if you have it installed.
-- Eden is also available as an [AUR package](https://aur.archlinux.org/packages/eden-git). If you are unable to build, either use that or compare your process to the PKGBUILD.
+Eden is also available as an [AUR package](https://aur.archlinux.org/packages/eden-git). If you are unable to build, either use that or compare your process to the PKGBUILD.
 
 ## Gentoo Linux
 
-Do not use the system sirit or xbyak packages.
+Enable the GURU repository to install [`games-emulation/eden`](https://gitweb.gentoo.org/repo/proj/guru.git/tree/games-emulation/eden). This repository also contains some additional dependencies, such as mcl, sirit, oaknut, etc.
 
 ## macOS
 
@@ -102,3 +101,60 @@ cmake -B build -DCMAKE_BUILD_TYPE=Release
 cmake --build build -- -j`nproc`
 cmake --install build
 ```
+
+## MSYS2
+
+`qt6-static` isn't supported yet.
+
+Only the `MINGW64` environment is tested; however, all of the others should work (in theory) sans `MINGW32`.
+
+Currently, only FFmpeg can be used as a system dependency; the others will result in linker errors.
+
+When packaging an MSYS2 build, you will need to copy all dependent DLLs recursively alongside the `windeployqt6`; for example:
+
+```sh
+# MSYS_TOOLCHAIN is typically just mingw64
+# since Windows is case-insensitive, you can set this to $MSYSTEM
+# or, if cross-compiling from Linux, set it to usr/x86_64-w64-mingw32
+export PATH="/${MSYS_TOOLCHAIN}/bin:$PATH"
+
+# grab deps of a dll or exe and place them in the current dir
+deps() {
+    # string parsing is fun
+    objdump -p "$1" | grep -e ".DLL Name:" | cut -d" " -f3 | while read -r dll; do
+        [ -z "$dll" ] && continue
+
+        # bin directory is used for DLLs, so we can do a quick "hack"
+        # and use command to find the path of the DLL
+        dllpath=$(command -v "$dll" 2>/dev/null || true)
+
+        [ -z "$dllpath" ] && continue
+
+        # explicitly include system32/syswow64 deps
+        # these aren't needed to be bundled, as all systems include them
+        case "$dllpath" in
+            *System32* | *SysWOW64*) continue ;;
+        esac
+
+        # avoid copying deps multiple times
+        if [ ! -f "$dll" ]; then
+            echo "$dllpath"
+            cp "$dllpath" "$dll"
+
+            # also grab the dependencies of the dependent DLL; e.g.
+            # double-conversion is a dep of Qt6Core.dll but NOT eden.exe
+            deps "$dllpath"
+        fi
+    done
+}
+
+# NB: must be done in a directory containing eden.exe
+deps eden.exe
+
+# deploy Qt plugins and such
+windeployqt6 --release --no-compiler-runtime \
+  --no-opengl-sw --no-system-dxc-compiler --no-system-d3d-compiler eden.exe
+
+# grab deps for Qt plugins
+find ./*/ -name "*.dll" | while read -r dll; do deps "$dll"; done
+```