MSVC 2015 compatibility: NaN is not constexpr.

It probably complains only because one of them (the float one) is
constexpr and the other is not. Whatever.

src/Magnum/Math/Test/ConstantsTest.cpp

@@ -78,7 +78,10 @@ template<class T> void ConstantsTest::_constants() {
 }
 
 template<class T> void ConstantsTest::_specials() {
-    constexpr T a = Constants<T>::nan();
+    #ifndef CORRADE_MSVC2015_COMPATIBILITY /* NaN is not constexpr */
+    constexpr
+    #endif
+    T a = Constants<T>::nan();
     CORRADE_VERIFY(std::isnan(a));
     CORRADE_VERIFY(a != a);