Do not pass empty object list to linker

This situation may arise in some situations with the compile cache
enabled. Some linkers (such as libtool on mac) throw an error if an
object list is empty, so it is not safe to pass it in.

tools/hxcpp/Linker.hx

@@ -251,7 +251,8 @@ class Linker
          }
 
          // Place list of obj files in a file called "all_objs"
-         if (mFromFile!="")
+         // In some situations with the compile cache, we might have an empty list
+         if (mFromFile!="" && objs.length!=0)
          {
             PathManager.mkdir(tmpDir);
             var fname = tmpDir + "/all_objs";