cxx_warnings: Misc changes to enable Werror

* CMake option to enable -Werror as default
    * Change to make sure all cxx examples have correct target name
    * Added PUBLIC CXX compiler flags to ads1x15 and ozw targets to
      work around compiler warnings
    * Renamed cmake variable for compiler warnings

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/lol/lol.cxx

@@ -130,6 +130,7 @@ void *do_draw(void *arg)
         }
 
     }
+    return NULL;
 }
 
 LoL::LoL() {