Math: don't allow *::data() to be called on rvalues.

There are more cases that should be fixed, but this is the most
problematic one, as this might look completely innocent:

    Vector2 a, b;
    float* c = (a + b).data();

Unlike this, which looks suspicious:

    Vector2& c = a + b;

src/Math/RectangularMatrix.h

@@ -164,8 +164,20 @@ template<std::size_t cols, std::size_t rows, class T> class RectangularMatrix {
          *
          * @see operator[]
          */
-        T* data() { return _data[0].data(); }
+        T* data()
-        constexpr const T* data() const { return _data[0].data(); } /**< @overload */
+        #ifndef CORRADE_GCC47_COMPATIBILITY
+        &
+        #endif
+        { return _data[0].data(); }
+
+        /** @overload */
+        constexpr const T* data()
+        #ifndef CORRADE_GCC47_COMPATIBILITY
+        const &
+        #else
+        const
+        #endif
+        { return _data[0].data(); }
 
         /**
          * @brief %Matrix column

src/Math/Vector.h

@@ -184,8 +184,20 @@ template<std::size_t size, class T> class Vector {
          *
          * @see operator[]()
          */
-        T* data() { return _data; }
+        T* data()
-        constexpr const T* data() const { return _data; } /**< @overload */
+        #ifndef CORRADE_GCC47_COMPATIBILITY
+        &
+        #endif
+        { return _data; }
+
+        /** @overload */
+        constexpr const T* data()
+        #ifndef CORRADE_GCC47_COMPATIBILITY
+        const &
+        #else
+        const
+        #endif
+        { return _data; }
 
         /**
          * @brief Value at given position