Add missing ASSIMP_BUILD_NO_C4D_IMPORTER check to C4DImporter.cpp. Fixes #503.

code/C4DImporter.cpp

@@ -42,6 +42,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *  @brief Implementation of the Cinema4D importer class.
  */
 #include "AssimpPCH.h"
+#ifndef ASSIMP_BUILD_NO_C4D_IMPORTER
 
 // no #ifdefing here, Cinema4D support is carried out in a branch of assimp
 // where it is turned on in the CMake settings. 
@@ -639,3 +640,5 @@ unsigned int C4DImporter::ResolveMaterial(PolygonObject* obj)
 	ai_assert((*it).second < mat_count);
 	return (*it).second;
 }
+
+#endif // ASSIMP_BUILD_NO_C4D_IMPORTER