Properly error out when using unsupported header.

src/BufferImage.h

@@ -144,6 +144,8 @@ template<UnsignedInt dimensions> inline BufferImage<dimensions>& BufferImage<dim
     std::swap(_buffer, other._buffer);
     return *this;
 }
+#else
+#error this header is available only on desktop OpenGL and OpenGL ES 3.0 build
 #endif
 
 }

src/BufferTexture.h

@@ -280,6 +280,8 @@ class MAGNUM_EXPORT BufferTexture: private AbstractTexture {
 };
 
 }
+#else
+#error this header is available only on desktop OpenGL build
 #endif
 
 #endif

src/CubeMapTextureArray.h

@@ -251,6 +251,8 @@ class CubeMapTextureArray: public AbstractTexture {
 };
 
 }
+#else
+#error this header is available only on desktop OpenGL build
 #endif
 
 #endif

src/ImageFormat.h

@@ -24,10 +24,12 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#ifdef MAGNUM_BUILD_DEPRECATED
 /** @file
  * @brief Enum @ref Magnum::ImageFormat, @ref Magnum::ImageType
  * @deprecated Use @ref ColorFormat.h instead.
  */
+#endif
 
 #include "ColorFormat.h"
 
@@ -48,7 +50,7 @@ typedef CORRADE_DEPRECATED("use ColorType instead") ColorType ImageType;
 
 }
 #else
-#error
+#error this header is available only on deprecated build
 #endif
 
 #endif

src/Swizzle.h

@@ -24,16 +24,17 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#ifdef MAGNUM_BUILD_DEPRECATED
 /** @file /Swizzle.h
  * @brief Function @ref Magnum::swizzle()
  * @deprecated Use @ref Math/Swizzle.h instead.
  */
+#endif
 
 #include "Math/Swizzle.h"
-
-#ifdef MAGNUM_BUILD_DEPRECATED
 #include "Color.h"
 
+#ifdef MAGNUM_BUILD_DEPRECATED
 namespace Magnum {
 
 /**
@@ -48,7 +49,7 @@ using Math::swizzle;
 
 }
 #else
-#error
+#error this header is available only on deprecated build
 #endif
 
 #endif