Normalize add_custom_command OUTPUT names (#10485)

Previously the OUTPUT arguments of add_custom_command were not
slash-normalized but those of add_library and add_executable were.
This caused the example

  add_custom_command(OUTPUT a//b.c ...)
  add_library(... a//b.c ...)

to fail at build time with "no rule to make a/b.c".  Fix this and modify
the CustomCommand test to try it.

Source/cmAddCustomCommandCommand.cxx

@@ -161,6 +161,7 @@ bool cmAddCustomCommandCommand
             filename += "/";
             }
           filename += copy;
+          cmSystemTools::ConvertToUnixSlashes(filename);
           break;
         case doing_source:
           // We do not want to convert the argument to SOURCE because