fixed very minor compil warnings. (no bugs)

common/base_screen.cpp

@@ -456,6 +456,7 @@ void BASE_SCREEN::AddGrid( const wxRealPoint& size, UserUnitType aUnit, int id )
         y = size.y / 25.4;
         break;
 
+    default:
     case INCHES:
     case UNSCALED_UNITS:
         x = size.x;

common/common.cpp

@@ -506,6 +506,7 @@ int From_User_Unit( UserUnitType aUnit, double val, int internal_unit_value )
         value = val * internal_unit_value;
         break;
 
+    default:
     case UNSCALED_UNITS:
         value = val;
     }

common/gr_basic.cpp

@@ -143,6 +143,8 @@ int GRMapY( int y )
 #define WHEN_INSIDE
 
 
+#if defined( USE_WX_ZOOM )
+// currently only used if USE_WX_ZOOM is defined.
 /**
  * Test if any part of a line falls within the bounds of a rectangle.
  *
@@ -373,6 +375,8 @@ static bool clipLine( EDA_Rect* aClipBox, int& x1, int& y1, int& x2, int& y2 )
 
     return true;
 }
+#endif      // if defined( USE_WX_ZOOM )
+
 
 /**
  * Function clip_line