Print error if transform feedback header is used in OpenGL ES 2.0 build.

src/Magnum/Context.cpp

@@ -50,7 +50,9 @@
 #include "Implementation/MeshState.h"
 #include "Implementation/ShaderProgramState.h"
 #include "Implementation/TextureState.h"
+#ifndef MAGNUM_TARGET_GLES2
 #include "Implementation/TransformFeedbackState.h"
+#endif
 
 namespace Magnum {
 

src/Magnum/Implementation/TransformFeedbackState.h

@@ -27,6 +27,10 @@
 
 #include "Magnum/Context.h"
 
+#ifdef MAGNUM_TARGET_GLES2
+#error This header is not available in OpenGL ES 2.0 build.
+#endif
+
 namespace Magnum { namespace Implementation {
 
 struct TransformFeedbackState {

src/Magnum/TransformFeedback.h

@@ -382,6 +382,8 @@ inline TransformFeedback& TransformFeedback::operator=(TransformFeedback&& other
 }
 
 }
+#else
+#error This header is not available in OpenGL ES 2.0 build.
 #endif
 
 #endif