Add Emscripten Build Instructions

Emscripten-build-instructions.md

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+# Wasm Build Instructions
+
+It should be noted that the wasm build of CppInterOp is still experimental and subject to change to change.  
+
+## CppInterOp Wasm Build Instructions
+
+This document first starts with the instructions on how to build a wasm build of CppInterOp. Before we start it should be noted that  
+unlike the non wasm version of CppInterOp we currently only support the Clang-REPL backend using llvm>19 for osx and Linux.  
+We will first make folder to build our wasm build of CppInterOp. This can be done by executing the following command
+
+```bash
+mkdir CppInterOp-wasm
+```
+
+Now move into this directory using the following command
+
+```bash
+cd ./CppInterOp-wasm
+```
+
+To create a wasm build of CppInterOp we make use of the emsdk toolchain. This can be installed by executing (we only currently  
+support version 3.1.45)
+```bash
+git clone https://github.com/emscripten-core/emsdk.git
+cd emsdk
+./emsdk install  3.1.45
+```
+
+Go back upto the top level build directory using
+
+```bash
+cd ../..
+```
+
+and activate the emsdk environment
+
+```bash
+./emsdk/emsdk activate 3.1.45
+source ./emsdk/emsdk_env.sh
+```
+
+Now clone the 19.x release of the LLVM project repository and CppInterOp (the building of the emscripten version of llvm can be
+avoided by executing micromamba install llvm -c <https://repo.mamba.pm/emscripten-forge> and setting the LLVM_BUILD_DIR appropriately)
+
+
+```bash
+git clone --depth=1 --branch release/19.x https://github.com/llvm/llvm-project.git
+git clone --depth=1 https://github.com/compiler-research/CppInterOp.git
+```
+
+Now move into the cloned llvm-project folder and apply a required patch
+
+```bash
+cd ./llvm-project/
+git apply -v ../CppInterOp/patches/llvm/emscripten-clang${{ matrix.clang-runtime }}-*.patch
+```
+
+We are now in a position to build an emscripten build of llvm by executing the following
+```bash
+mkdir build
+cd build
+emcmake cmake -DCMAKE_BUILD_TYPE=Release \
+                        -DLLVM_HOST_TRIPLE=wasm32-unknown-emscripten \
+                        -DLLVM_ENABLE_ASSERTIONS=ON                        \
+                        -DLLVM_TARGETS_TO_BUILD="WebAssembly" \
+                        -DLLVM_ENABLE_LIBEDIT=OFF \
+                        -DLLVM_ENABLE_PROJECTS="clang;lld" \
+                        -DLLVM_ENABLE_ZSTD=OFF \
+                        -DLLVM_ENABLE_LIBXML2=OFF \
+                        -DCLANG_ENABLE_STATIC_ANALYZER=OFF \
+                        -DCLANG_ENABLE_ARCMT=OFF \
+                        -DCLANG_ENABLE_BOOTSTRAP=OFF \
+                        -DCMAKE_CXX_FLAGS="-Dwait4=__syscall_wait4" \
+                        ../llvm
+emmake make clang -j $(nproc --all)
+emmake make clang-repl -j $(nproc --all)
+emmake make lld -j $(nproc --all)
+```
+
+Once this finishes building we need to take note of where we built our llvm build. This can be done by executing the following
+
+```bash
+export LLVM_BUILD_DIR=$PWD
+```
+
+We can move onto building the wasm  
+version of CppInterOp. To do this execute the following  
+
+```bash
+cd ../../CppInterOp/
+mkdir build
+cd ./build/
+emcmake cmake -DCMAKE_BUILD_TYPE=Release    \
+                -DUSE_CLING=OFF                             \
+                -DUSE_REPL=ON                               \
+                -DLLVM_DIR=$LLVM_BUILD_DIR/lib/cmake/llvm      \
+                -DLLD_DIR=$LLVM_BUILD_DIR/lib/cmake/lld     \
+                -DClang_DIR=$LLVM_BUILD_DIR/lib/cmake/clang     \
+                -DBUILD_SHARED_LIBS=ON                      \
+                -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ON            \
+                -DCMAKE_INSTALL_PREFIX=$PREFIX         \
+                ../
+emmake make -j $(nproc --all)
+```bash
+
+Once this finishes building we need to take note of where we built our llvm build. This can be done by executing the following
+
+```bash
+export CPPINTEROP_BUILD_DIR=$PWD
+```
+
+## Xeus-cpp-lite Wasm Build Instructions
+
+A project which makes use of the wasm build of CppInterOp is xeus-cpp. xeus-cpp is a C++ Jupyter kernel. To build xeus-cpp we first need to  
+install several other dependencies. To create an Conda environment with this dependencies installed execute the following from the  
+CppInterOp build folder (these instructions will assume you have already installed micromamba)  
+
+```bash
+cd ..
+micromamba create -f environment-wasm.yml --platform=emscripten-wasm32
+micromamba activate CppInterOp-wasm
+export PREFIX=$MAMBA_ROOT_PREFIX/envs/CppInterOp-wasm
+```
+Now we can build an wasm build of xeus-cpp by executing the following  
+
+```bash
+cd ..
+git clone --depth=1 https://github.com/compiler-research/xeus-cpp.git
+cd ./xeus-cpp
+mkdir build
+cd build
+export CMAKE_PREFIX_PATH=$PREFIX
+export CMAKE_SYSTEM_PREFIX_PATH=$PREFIX 
+emcmake cmake \
+          -DCMAKE_BUILD_TYPE=Release                                     \
+          -DCMAKE_PREFIX_PATH=$PREFIX                                    \
+          -DCMAKE_INSTALL_PREFIX=$PREFIX                                 \
+          -DXEUS_CPP_EMSCRIPTEN_WASM_BUILD=ON                            \
+          -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ON                         \
+          -DCppInterOp_DIR="$CPPINTEROP_BUILD_DIR/lib/cmake/CppInterOp"  \
+          ..
+ emmake make -j $(nproc --all) install
+```
+
+To build Jupyter Lite with this kernel without creating a website you can execute the following
+
+```bash
+cd ../..
+micromamba create -n xeus-lite-host jupyterlite-core
+micromamba activate xeus-lite-host
+python -m pip install jupyterlite-xeus
+jupyter lite build --XeusAddon.prefix=$PREFIX
+```
+
+Once the Jupyter Lite site has built you can test the website locally  by executing
+
+```bash
+jupyter lite serve --XeusAddon.prefix=$PREFIX
+```