Create option to build against a CMake build of LLVM
Set `PREBUILT_LLVM_PATH` to the root of an LLVM tree built with CMake
where the build output directory is named `build`.
The path can be provided relative to the root of the Crubit tree.
PiperOrigin-RevId: 450711879
diff --git a/README.md b/README.md
index 55b3068..671119b 100644
--- a/README.md
+++ b/README.md
@@ -13,4 +13,16 @@
$ git clone git@github.com:google/crubit.git
$ cd crubit
$ bazel build --linkopt=-fuse-ld=/usr/bin/ld.lld //rs_bindings_from_cc:rs_bindings_from_cc_impl
+```
+
+### Using a prebuilt LLVM tree
+
+```
+$ git clone https://github.com/llvm/llvm-project
+$ cd llvm-project
+$ CC=clang CXX=clang++ cmake -S llvm -B build -DLLVM_ENABLE_PROJECTS='clang' -DCMAKE_BUILD_TYPE=Release
+$ cmake --build build -j
+$ # wait...
+$ cd ../crubit
+$ PREBUILT_LLVM_PATH=../llvm-project bazel build //rs_bindings_from_cc:rs_bindings_from_cc_impl
```
\ No newline at end of file
