vendor/box2d: clarify wasm building

vendor/box2d/wasm.Makefile

@@ -2,6 +2,8 @@
 # I tried to make a cmake toolchain file for this / use cmake but this is far easier.
 # NOTE: We are pretending to be emscripten to box2d so it takes WASM code paths, but we don't actually use emscripten.
 
+# WARN: wasm is probably not supported by your default C compiler and linker, overwrite the CC and LD environment variables accordingly.
+# Example for MacOS:
 # CC = $(shell brew --prefix llvm)/bin/clang
 # LD = $(shell brew --prefix llvm)/bin/wasm-ld