GCC 4.5 compatibility: can't `default` an `explicit` declaration.

src/Shapes/shapeImplementation.h

@@ -132,7 +132,7 @@ template<UnsignedInt dimensions> struct MAGNUM_SHAPES_EXPORT AbstractShape {
 template<class T> struct Shape: AbstractShape<T::Dimensions> {
     T shape;
 
-    explicit Shape() = default;
+    explicit Shape();
     explicit Shape(const T& shape): shape(shape) {}
     explicit Shape(T&& shape): shape(std::move(shape)) {}
 
@@ -150,6 +150,8 @@ template<class T> struct Shape: AbstractShape<T::Dimensions> {
     }
 };
 
+template<class T> Shape<T>::Shape() = default;
+
 }}}
 
 #endif