Fix macOS link: use ld64 flags instead of GNU ld ones

ld64.lld rejects --allow-shlib-undefined and -rpath=$ORIGIN. Add a
macOS branch with -undefined dynamic_lookup and an @executable_path
rpath, and skip the Linux-only patchelf step in the justfile.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

justfile

@@ -28,10 +28,15 @@ run-webcam-demo: build-webcam-demo
 _build binary-name:
     lake build {{binary-name}} --verbose
 
+[linux]
 _perform_nix_patch binary-name:
     patchelf --set-interpreter /usr/lib/ld-linux-x86-64.so.2 .lake/build/bin/{{binary-name}}
     patchelf --set-rpath '$ORIGIN:/usr/lib' .lake/build/bin/{{binary-name}}
 
+[macos]
+_perform_nix_patch binary-name:
+    @true
+
 
 # Build the C version of the SDL3 webcam
 [group: "C_build"]

lakefile.lean

@@ -144,9 +144,12 @@ def sdlLinkLibs : TargetArray Dynlib := #[libSDL3, libSDL3Image, libSDL3Ttf, lib
 
 -- make sure to copy these link args into whatever project is using this library in order for it to work
 -- This is because without "-rpath=$ORIGIN", the Linux executable will not load dynlibs next to the executable (i.e., the SDL ones you've copied there).
-def sdlLinkArgs : Array String := if Platform.isWindows
-  then
+-- macOS's ld64 spells both flags differently: `-undefined dynamic_lookup` and `@executable_path` in place of `$ORIGIN`.
+def sdlLinkArgs : Array String :=
+  if Platform.isWindows then
     #[]
+  else if Platform.isOSX then
+    #["-Wl,-undefined,dynamic_lookup", "-Wl,-rpath,@executable_path"]
   else
     #["-Wl,--allow-shlib-undefined", "-Wl,-rpath=$ORIGIN"]