Add support for macOS

vendor/commonmark/cmark.odin

@@ -23,6 +23,8 @@ when ODIN_OS == .Windows {
 	}
 } else when ODIN_OS == .Linux {
 	foreign import lib "system:cmark"
+} else when ODIN_OS == .Darwin {
+	foreign import lib "system:cmark"
 }
 
 Option :: enum c.int {
@@ -529,4 +531,4 @@ get_default_mem_allocator_as_odin :: proc() -> runtime.Allocator {
 		procedure = cmark_allocator_proc,
 		data = rawptr(get_default_mem_allocator()),
 	}
-}
+}