charmpp: default backend to 'multicore' on macOS (#28443)

var/spack/repos/builtin/packages/charmpp/package.py

@@ -69,9 +69,15 @@ class Charmpp(Package):
     )
 
     # Communication mechanisms (choose exactly one)
+    # - Default to 'multicore' on macOS because that's probably the right choice
+    #   for a personal machine.
+    if sys.platform == "darwin":
+        backend_default = "multicore"
+    else:
+        backend_default = "netlrts"
     variant(
         "backend",
-        default="netlrts",
+        default=backend_default,
         values=("mpi", "multicore", "netlrts", "verbs", "gni",
                 "ucx", "ofi", "pami", "pamilrts"),
         description="Set the backend to use"