DevIL library: save gray scale image in three matrices instead one

I need to make a program that convert a RGB image to a GRAYSCALE image and save it in PGM format. I use DevIL library, but when I save the image, I obtain always a 3D image (3 matrix), in grayscale but, if I load it in MATLAB, I have 3 matrices instead of just one. How can I obtain just one matrix in my output file using DevIL?

 int main()
 {

  ilInit();
  ilEnable(IL_ORIGIN_SET);
  ilOriginFunc(IL_ORIGIN_UPPER_LEFT);
  ilEnable(IL_FILE_OVERWRITE);

  ILuint ImageName; // The image name to return.
  ilGenImages(1, &ImageName);
  ilBindImage(ImageName);
  if(!ilLoadImage("/home/andrea/Scrivania/tests/siftDemoV4/et000.jpg"))
  {   printf("err");
      exit;
  }
  else
    printf("caricata\n");

  ILuint width,height;
  width = ilGetInteger(IL_IMAGE_WIDTH);
  height = ilGetInteger(IL_IMAGE_HEIGHT);
  double v[3]={0.2989360212937755001405548682669177651405334472656250000,0.5870430744511212495240215503145009279251098632812500000,0.1140209042551033058465748126764083281159400939941406250};
 printf("%.55f %.55f %.55f",v[0],v[1],v[2]);
ILubyte *imgValue=ilGetData();
int i=0;
ILubyte imgNuova[width*height];


while( i < width*height)
{


        imgNuova[i]=(char)round( ( (double)imgValue[3*i]*v[0])+ ( (double)imgValue[3*i+1]*v[1])+((double)imgValue[3*i+2]*v[2]));

        i++;

}


ILuint ImageName2;

ilGenImages(2, &ImageName2);
ilBindImage(ImageName2);
ilTexImage(width, height, 1, 1, IL_LUMINANCE,
           IL_UNSIGNED_BYTE, imgNuova);
iluFlipImage();
ilSave(IL_PNM,"/home/andrea/Scrivania/tests/siftDemoV4/et000new.pgm");

return 0;

}

Unfortunately, due to a bug in the PNM export, DevIL can and will only write PPM (Portable Pixmaps, 3 channel RGB) files regardless of the file extension. The only solution to this is to use a different file format, that supports single channel grayscale images, like PNG.

Matlab should be able to use that just as well. If you absolutely need or want files in the PGM format, you will have to use a converter like png2pnm.