Makefile: Fix "make clean" when there are no targets

The empty `clean` target was missing from `Makefile2`.
Add one like we already have for `all` and `preinstall`.

Source/cmGlobalUnixMakefileGenerator3.cxx

@@ -241,6 +241,10 @@ void cmGlobalUnixMakefileGenerator3::WriteMainMakefile2()
   lg->WriteMakeRule(makefileStream, "The main recursive preinstall target",
                     "preinstall", depends, no_commands, true);
 
+  // Write an empty clean:
+  lg->WriteMakeRule(makefileStream, "The main recursive clean target", "clean",
+                    depends, no_commands, true);
+
   // Write out the "special" stuff
   lg->WriteSpecialTargetsTop(makefileStream);