Fix linking of tests on MinGW-w64 GCC 5.1.

It worked flawlessly when crosscompiled from Linux, but compiling that
natively causes the linker to loudly complain about undefined references
to ConfigurationValue structs. I think it worked well under 4.9.2 (but
that mess had a slew of other ugly problems, such as complete inability
to produce non-crashing C++11 code under x64). The linker error is
caused only because I tried to reduce binary bloat, so I'm just
disabling that for MinGW and screw that. No problem under MSVC.

src/Magnum/Math/Range.h

@@ -580,7 +580,7 @@ template<class T> struct ConfigurationValue<Magnum::Math::Range2D<T>>: public Co
 /** @configurationvalue{Magnum::Math::Range3D} */
 template<class T> struct ConfigurationValue<Magnum::Math::Range3D<T>>: public ConfigurationValue<Magnum::Math::Range<3, T>> {};
 
-#ifndef DOXYGEN_GENERATING_OUTPUT
+#if !defined(DOXYGEN_GENERATING_OUTPUT) && !defined(__MINGW32__)
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::Range<2, Magnum::Float>>;
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::Range<2, Magnum::Int>>;
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::Range<3, Magnum::Float>>;

src/Magnum/Math/RectangularMatrix.h

@@ -736,7 +736,7 @@ template<std::size_t cols, std::size_t rows, class T> struct ConfigurationValue<
     }
 };
 
-#ifndef DOXYGEN_GENERATING_OUTPUT
+#if !defined(DOXYGEN_GENERATING_OUTPUT) && !defined(__MINGW32__)
 /* Square matrices */
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::RectangularMatrix<2, 2, Magnum::Float>>;
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::RectangularMatrix<3, 3, Magnum::Float>>;

src/Magnum/Math/Vector.h

@@ -1400,7 +1400,7 @@ template<std::size_t size, class T> struct ConfigurationValue<Magnum::Math::Vect
     }
 };
 
-#ifndef DOXYGEN_GENERATING_OUTPUT
+#if !defined(DOXYGEN_GENERATING_OUTPUT) && !defined(__MINGW32__)
 /* Vectors */
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::Vector<2, Magnum::Float>>;
 extern template struct MAGNUM_EXPORT ConfigurationValue<Magnum::Math::Vector<3, Magnum::Float>>;