GCC 4.6 compatibility: no delegating constructors.

Don't know why the corresponding conversion operator can't be constexpr.

src/Math/Test/VectorTest.cpp

@@ -201,10 +201,18 @@ void VectorTest::convert() {
     constexpr Vec3 a{1.5f, 2.0f, -3.5f};
     constexpr Vector3 b(1.5f, 2.0f, -3.5f);
 
+    #ifndef CORRADE_GCC46_COMPATIBILITY
     constexpr Vector3 c(a);
+    #else
+    Vector3 c(a); /* Not constexpr under GCC < 4.7 */
+    #endif
     CORRADE_COMPARE(c, b);
 
+    #ifndef CORRADE_GCC46_COMPATIBILITY
     constexpr Vec3 d(b);
+    #else
+    Vec3 d(b); /* Not constexpr under GCC < 4.7 */
+    #endif
     CORRADE_COMPARE(d.x, a.x);
     CORRADE_COMPARE(d.y, a.y);
     CORRADE_COMPARE(d.z, a.z);

src/Math/Vector.h

@@ -162,7 +162,13 @@ template<std::size_t size, class T> class Vector {
         #endif
 
         /** @brief Construct vector from external representation */
+        #ifndef CORRADE_GCC46_COMPATIBILITY
         template<class U, class V = decltype(Implementation::VectorConverter<size, T, U>::from(std::declval<U>()))> inline constexpr explicit Vector(const U& other): Vector(Implementation::VectorConverter<size, T, U>::from(other)) {}
+        #else
+        template<class U, class V = decltype(Implementation::VectorConverter<size, T, U>::from(std::declval<U>()))> inline explicit Vector(const U& other) {
+            *this = Vector(Implementation::VectorConverter<size, T, U>::from(other));
+        }
+        #endif
 
         /** @brief Copy constructor */
         inline constexpr Vector(const Vector<size, T>&) = default;
@@ -172,6 +178,7 @@ template<std::size_t size, class T> class Vector {
 
         /** @brief Convert vector to external representation */
         template<class U, class V = decltype(Implementation::VectorConverter<size, T, U>::to(std::declval<Vector<size, T>>()))> inline constexpr explicit operator U() const {
+            /** @bug Why this is not constexpr under GCC 4.6? */
             return Implementation::VectorConverter<size, T, U>::to(*this);
         }