Added missing docs/build.md. (0.38.15)

docs/build.md

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+# Building Fun
+
+This guide describes how to build Fun from source using CMake and the available build options.
+
+## Prerequisites
+- A C compiler with C99 support
+- CMake 3.20+ (or newer)
+- Optional: Rust toolchain with cargo (required when building with `FUN_WITH_RUST=ON`)
+
+## Common targets
+- `build` - aggregate target that depends on `fun`, `fun_test`, and `test_opcodes`
+- `fun` - the CLI executable
+- `fun_test` - unit/feature tests (run with CTest)
+- `test_opcodes` - opcode tests (executable)
+
+These targets are defined by the project; use your configured CMake build directory/profile.
+
+## Build options
+Fun exposes several options you can toggle at configure time:
+
+- `FUN_DEBUG` (ON/OFF) - Enables extra assertions and logging in the VM and runtime
+- `FUN_USE_MUSL` (ON/OFF) - Link against musl for static/portable builds (Linux)
+- `FUN_WITH_CPP` (ON/OFF) - Enable C++-based opcode/examples support
+- `FUN_WITH_RUST` (ON/OFF) - Build and link Rust staticlib from `src/rust/`
+- `FUN_WITH_OPENSSL` (ON/OFF) - Enable OpenSSL-backed helpers (MD5/SHA-256/SHA-512/RIPEMD-160)
+
+When configuring, the build prints a summary like:
+
+```
+==== Fun build options ====
+  FUN_DEBUG: ENABLED|DISABLED
+  FUN_USE_MUSL: ENABLED|DISABLED
+  FUN_WITH_CPP: ENABLED|DISABLED
+  FUN_WITH_RUST: ENABLED|DISABLED
+===========================
+```
+
+## Example commands
+Use the CLion-provided build directories or your own. Typical invocations:
+
+### Debug
+```
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug \
+  -DFUN_DEBUG=ON -DFUN_WITH_RUST=OFF
+cmake --build build --target build
+```
+
+### Release
+```
+cmake -S . -B build_release -DCMAKE_BUILD_TYPE=Release \
+  -DFUN_DEBUG=OFF -DFUN_WITH_RUST=OFF
+cmake --build build_release --target build
+```
+
+### Enabling optional extensions
+```
+cmake -S . -B build_release -DCMAKE_BUILD_TYPE=Release \
+  -DFUN_WITH_CPP=ON -DFUN_WITH_RUST=ON -DFUN_WITH_OPENSSL=ON
+cmake --build build_release --target build
+```
+
+If `FUN_WITH_RUST` is enabled, ensure `cargo` is available in PATH; the build will invoke it and link the produced static library.
+
+If `FUN_WITH_OPENSSL` is enabled, CMake must detect your system OpenSSL. Hash helpers use the EVP interface (OpenSSL 3-compatible). Note: RIPEMD-160 may require the legacy provider on OpenSSL 3.x; if unavailable, the helper returns an empty string.
+
+## Running
+- CLI: run the `fun` executable from your build directory.
+- REPL: `fun -i` or just run `fun` without a script, depending on your CLI version (see [cli.md](./cli.md)).
+- Examples: see [examples.md](./examples.md).
+
+Tip: When running from the repository without installation, set `FUN_LIB_DIR` to the local `./lib` so includes can find the stdlib.