singles: add include guards for the implementation parts.

With seemingly innocent code like below, it happened that the
implementation part from MagnumMath.hpp got included twice because
MagnumMathBatch.hpp includes MagnumMath.hpp as a dependency, leading to
duplicate symbol definitions. This should be a completely valid use
case, so I'm adding guards to ensure the implementation is always only
added if not already.

    #define MAGNUM_MATH_IMPLEMENTATION
    #include <MagnumMath.hpp>
    #include <MagnumMathBatch.hpp>

A corresponding test case is going to get added to the singles repo.

src/singles/MagnumMath.hpp

@@ -447,7 +447,17 @@ typedef Math::Frustum<Double> Frustumd;
 // TODO: DynamicMatrixIntegration (separate library because of StridedArrayView)
 #include "Magnum/EigenIntegration/GeometryIntegration.h"
 #endif
-#ifdef MAGNUM_MATH_IMPLEMENTATION
+/* The extra guard has to be here to prevent double definitions in cases like
+
+    #define MAGNUM_MATH_IMPLEMENTATION
+    #include <MagnumMath.hpp>
+    #include <MagnumMathBatch.hpp>
+
+   where MagnumMathBatch.hpp contains `#include <MagnumMath.hpp>` again. Note
+   that even the stb_* libs don't handle this -- including them twice with the
+   implementation macro defined *will* lead to double definitions. */
+#if defined(MAGNUM_MATH_IMPLEMENTATION) && !defined(MagnumMath_hpp_implementation)
+#define MagnumMath_hpp_implementation
 // {{ includes }}
 #include "Magnum/Math/Functions.cpp"
 #include "Magnum/Math/Packing.cpp"

src/singles/MagnumMathBatch.hpp

@@ -67,6 +67,17 @@
 // TODO ColorBatch once it's more useful
 #include "Magnum/Math/FunctionsBatch.h"
 #include "Magnum/Math/PackingBatch.h"
-#ifdef MAGNUM_MATH_BATCH_IMPLEMENTATION
+/* The extra guard has to be here to prevent double definitions in cases like
+
+    #define MAGNUM_MATH_BATCH_IMPLEMENTATION
+    #include <MagnumMathBatch.hpp>
+    #include <SomeOtherLib.h>
+
+   where a hypothetical SomeOtherLib.h contains `#include <MagnumMathBatch.hpp>`
+   again. Note that even the stb_* libs don't handle this -- including them
+   twice with the implementation macro defined *will* lead to double
+   definitions. */
+#if defined(MAGNUM_MATH_BATCH_IMPLEMENTATION) && !defined(MagnumMathBatch_hpp_implementation)
+#define MagnumMathBatch_hpp_implementation
 #include "Magnum/Math/PackingBatch.cpp"
 #endif

src/singles/MagnumMeshTools.hpp

@@ -109,7 +109,18 @@ template<class T> using Array4 = StaticArray<4, T>;
 // TODO FlipNormals?
 // TODO GenerateNormals?
 // TODO RemoveDuplicates?
-#ifdef MAGNUM_MESHTOOLS_IMPLEMENTATION
+/* The extra guard has to be here to prevent double definitions in cases like
+
+    #define MAGNUM_MESHTOOLS_IMPLEMENTATION
+    #include <MagnumMeshTools.hpp>
+    #include <SomeOtherLib.h>
+
+   where a hypothetical SomeOtherLib.h contains `#include <MagnumMeshTools.hpp>`
+   again. Note that even the stb_* libs don't handle this -- including them
+   twice with the implementation macro defined *will* lead to double
+   definitions. */
+#if defined(MAGNUM_MESHTOOLS_IMPLEMENTATION) && !defined(MagnumMeshTools_hpp_implementation)
+#define MagnumMeshTools_hpp_implementation
 // {{includes}}
 #if !defined(CORRADE_ASSERT_UNREACHABLE) && !defined(NDEBUG)
 #include <cassert>