build: add lua-cjson submodule and fix make bench for macOS

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "vendor/lua-cjson"]
+	path = vendor/lua-cjson
+	url = https://github.com/openresty/lua-cjson.git

Makefile

@@ -1,9 +1,16 @@
 # Overridable: `make bench LUAJIT=/path/to/luajit LUA_CPATH='...'`
 LUAJIT    ?= $(shell command -v luajit 2>/dev/null || echo /usr/local/openresty/luajit/bin/luajit)
-LUA_CPATH ?= ./?.so;/usr/local/openresty/lualib/?.so;/usr/local/lib/lua/5.1/?.so;/usr/local/openresty/luajit/lib/lua/5.1/?.so
+LUA_CPATH ?= ./vendor/lua-cjson/?.so;./?.so;/usr/local/openresty/lualib/?.so;/usr/local/lib/lua/5.1/?.so;/usr/local/openresty/luajit/lib/lua/5.1/?.so
+
+LUAJIT_PREFIX ?= $(shell dirname $$(dirname $$(command -v $(LUAJIT) 2>/dev/null || echo /usr/local/openresty/luajit/bin/luajit)))
+LUAJIT_INC    ?= $(LUAJIT_PREFIX)/include/luajit-2.1
 
 LIB_DIR := $(CURDIR)/target/release
+ifeq ($(shell uname),Darwin)
+LUA_ENV := DYLD_LIBRARY_PATH=$(LIB_DIR) LUA_CPATH='$(LUA_CPATH)'
+else
 LUA_ENV := LD_LIBRARY_PATH=$(LIB_DIR) LUA_CPATH='$(LUA_CPATH)'
+endif
 
 .PHONY: help build test lint bench clean
 
@@ -22,8 +29,15 @@ test: build ## Run cargo tests + busted Lua tests
 lint: ## Run clippy with -D warnings
 	cargo clippy --release --all-targets -- -D warnings
 
-bench: build ## Run the LuaJIT vs cjson benchmark
+bench: build vendor/lua-cjson/cjson.so ## Run the LuaJIT vs cjson benchmark
 	$(LUA_ENV) $(LUAJIT) benches/lua_bench.lua
 
+vendor/lua-cjson/cjson.so: | vendor/lua-cjson/Makefile
+ifeq ($(shell uname),Darwin)
+	$(MAKE) -C vendor/lua-cjson PREFIX=$(LUAJIT_PREFIX) LUA_INCLUDE_DIR=$(LUAJIT_INC) LUA=$(LUAJIT) CJSON_LDFLAGS="-bundle -undefined dynamic_lookup"
+else
+	$(MAKE) -C vendor/lua-cjson PREFIX=$(LUAJIT_PREFIX) LUA_INCLUDE_DIR=$(LUAJIT_INC) LUA=$(LUAJIT)
+endif
+
 clean: ## Remove build artifacts
 	cargo clean