add linux clang instructions

COMPILE.md

@@ -72,7 +72,119 @@ cd .\build\windows64\Minecraft.Server\Debug
 ```
 
 Notes:
-- The CMake build is Windows-only and x64-only.
-- Contributors on macOS or Linux need a Windows machine or VM to build the project. Running the game via Wine is separate from having a supported build environment.
 - Post-build asset copy is automatic for `Minecraft.Client` in CMake (Debug and Release variants).
 - The game relies on relative paths (for example `Common\Media\...`), so launching from the output directory is required.
+
+## CMake (Linux x64 Cross-Compile with Clang)
+
+Cross-compile Windows x64 binaries on Linux using LLVM/Clang and the Windows SDK obtained via xwin.
+
+### Prerequisites
+
+Install the following packages (example for Ubuntu):
+
+```bash
+sudo apt install clang lld llvm cmake ninja-build rsync cargo
+```
+
+Install xwin for downloading the Windows SDK:
+
+```bash
+cargo install xwin
+```
+
+### Download Windows SDK
+
+Download and extract the Windows SDK and CRT:
+
+```bash
+xwin --accept-license splat --output ~/.cache/xwin/splat
+```
+
+Create symlinks to account for Linux filesystems being case sensitive:
+
+```bash
+WINSDK=~/.cache/xwin/splat
+ln -sf $WINSDK/sdk/include/shared/sdkddkver.h $WINSDK/sdk/include/shared/SDKDDKVer.h
+ln -sf $WINSDK/sdk/lib/um/x86_64/xinput9_1_0.lib $WINSDK/sdk/lib/um/x86_64/XInput9_1_0.lib
+ln -sf $WINSDK/sdk/lib/um/x86_64/ws2_32.lib $WINSDK/sdk/lib/um/x86_64/Ws2_32.lib
+```
+
+### Configure
+
+Set environment variables and configure CMake:
+
+```bash
+export WINSDK=~/.cache/xwin/splat
+export INCLUDE="$WINSDK/crt/include;$WINSDK/sdk/include/um;$WINSDK/sdk/include/ucrt;$WINSDK/sdk/include/shared"
+export LIB="$WINSDK/crt/lib/x86_64;$WINSDK/sdk/lib/um/x86_64;$WINSDK/sdk/lib/ucrt/x86_64"
+
+cmake -S . -B build/windows64-clang \
+  -G Ninja \
+  -DCMAKE_BUILD_TYPE=Release \
+  -DCMAKE_C_COMPILER=clang-cl \
+  -DCMAKE_CXX_COMPILER=clang-cl \
+  -DCMAKE_LINKER=lld-link \
+  -DCMAKE_RC_COMPILER=llvm-rc \
+  -DCMAKE_MT=llvm-mt \
+  -DPLATFORM_DEFINES="_WINDOWS64" \
+  -DPLATFORM_NAME="Windows64" \
+  -DIGGY_LIBS="iggy_w64.lib;iggyperfmon_w64.lib;iggyexpruntime_w64.lib" \
+  -DCMAKE_SYSTEM_NAME=Windows \
+  -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded \
+  -DCMAKE_C_FLAGS="/MT -fms-compatibility -fms-extensions --target=x86_64-pc-windows-msvc -imsvc $WINSDK/crt/include -imsvc $WINSDK/sdk/include/ucrt -imsvc $WINSDK/sdk/include/um -imsvc $WINSDK/sdk/include/shared" \
+  -DCMAKE_CXX_FLAGS="/MT -fms-compatibility -fms-extensions --target=x86_64-pc-windows-msvc -imsvc $WINSDK/crt/include -imsvc $WINSDK/sdk/include/ucrt -imsvc $WINSDK/sdk/include/um -imsvc $WINSDK/sdk/include/shared" \
+  -DCMAKE_ASM_MASM_FLAGS="-m64" \
+  -DCMAKE_EXE_LINKER_FLAGS="-libpath:$WINSDK/crt/lib/x86_64 -libpath:$WINSDK/sdk/lib/um/x86_64 -libpath:$WINSDK/sdk/lib/ucrt/x86_64"
+```
+
+### Build
+
+Build Release:
+
+```bash
+cmake --build build/windows64-clang --config Release
+```
+
+Build specific target:
+
+```bash
+cmake --build build/windows64-clang --config Release --target Minecraft.Client
+cmake --build build/windows64-clang --config Release --target Minecraft.Server
+```
+
+### Run with Wine
+
+Run executable:
+
+```bash
+cd build/windows64-clang/Minecraft.Client
+wine ./Minecraft.Client.exe
+```
+
+Run dedicated server:
+
+```bash
+cd build/windows64-clang/Minecraft.Server
+wine ./Minecraft.Server.exe -port 25565 -bind 0.0.0.0 -name DedicatedServer
+```
+
+### NixOS / Nix
+
+For NixOS or systems with Nix installed, use the provided flake:
+
+```bash
+nix build .#client
+nix build .#server
+```
+
+Or enter the development shell with all dependencies:
+
+```bash
+nix develop
+```
+
+Notes:
+- Requires LLVM 15+ with clang-cl, lld-link, llvm-rc, and llvm-mt.
+- The xwin tool downloads ~1GB of SDK files on first run.
+- Wine is required to run the compiled Windows executables on Linux.