Don't allow 12-bit JPEG support to be disabled

In libjpeg-turbo 2.1.x and prior, the WITH_12BIT CMake variable was used
to enable 12-bit JPEG support at compile time, because the libjpeg API
library could not handle multiple JPEG data precisions at run time.  The
initial approach to handling multiple JPEG data precisions at run time
(7fec5074f9) created a whole new API,
library, and applications for 12-bit data precision, so it made sense to
repurpose WITH_12BIT to allow 12-bit data precision to be disabled.
e8b40f3c2b made it so that the libjpeg API
library can handle multiple JPEG data precisions at run time via a
handful of straightforward API extensions.  Referring to
6c2bc901e2, it hasn't been possible to
build libjpeg-turbo with both forward and backward libjpeg API/ABI
compatibility since libjpeg-turbo 1.4.x.  Thus, whereas we retain full
backward API/ABI compatibility with libjpeg v6b-v8, forward libjpeg
API/ABI compatibility ceased being realistic years ago, so it no longer
makes sense to provide compile-time options that give a false sense of
forward API/ABI compatibility by allowing some (but not all) of our
libjpeg API extensions to be disabled.  Such options are difficult to
maintain and clutter the code with #ifdefs.

example.c

@@ -114,12 +114,10 @@ write_JPEG_file(char *filename, int quality, int data_precision)
   JSAMPARRAY image_buffer = NULL;
                                 /* Points to large array of R,G,B-order data */
   JSAMPROW row_pointer[1];      /* pointer to JSAMPLE row[s] */
-#ifdef WITH_12BIT
   J12SAMPARRAY image_buffer12 = NULL;
                                 /* Points to large array of R,G,B-order 12-bit
                                    data */
   J12SAMPROW row_pointer12[1];  /* pointer to J12SAMPLE row[s] */
-#endif
   int row_stride;               /* physical row width in image buffer */
   int row, col;
 
@@ -183,7 +181,6 @@ write_JPEG_file(char *filename, int quality, int data_precision)
    * sample array and initialize it for each successive scanline written in the
    * scanline loop below.
    */
-#ifdef WITH_12BIT
   if (cinfo.data_precision == 12) {
     image_buffer12 = (J12SAMPARRAY)(*cinfo.mem->alloc_sarray)
       ((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, HEIGHT);
@@ -200,9 +197,7 @@ write_JPEG_file(char *filename, int quality, int data_precision)
            col * (MAXJ12SAMPLE + 1) / WIDTH) % (MAXJ12SAMPLE + 1);
       }
     }
-  } else
-#endif
-  {
+  } else {
     image_buffer = (*cinfo.mem->alloc_sarray)
       ((j_common_ptr)&cinfo, JPOOL_IMAGE, row_stride, HEIGHT);
 
@@ -227,7 +222,6 @@ write_JPEG_file(char *filename, int quality, int data_precision)
    * To keep things simple, we pass one scanline per call; you can pass
    * more if you wish, though.
    */
-#ifdef WITH_12BIT
   if (cinfo.data_precision == 12) {
     while (cinfo.next_scanline < cinfo.image_height) {
       /* jpeg12_write_scanlines expects an array of pointers to scanlines.
@@ -237,9 +231,7 @@ write_JPEG_file(char *filename, int quality, int data_precision)
       row_pointer12[0] = image_buffer12[cinfo.next_scanline];
       (void)jpeg12_write_scanlines(&cinfo, row_pointer12, 1);
     }
-  } else
-#endif
-  {
+  } else {
     while (cinfo.next_scanline < cinfo.image_height) {
       /* jpeg_write_scanlines expects an array of pointers to scanlines.
        * Here the array is only one element long, but you could pass
@@ -403,10 +395,8 @@ do_read_JPEG_file(struct jpeg_decompress_struct *cinfo, char *infilename,
   FILE *infile;                 /* source file */
   FILE *outfile;                /* output file */
   JSAMPARRAY buffer;            /* Output row buffer */
-#ifdef WITH_12BIT
   J12SAMPARRAY buffer12;        /* 12-bit output row buffer */
   int col;
-#endif
   int row_stride;               /* physical row width in output buffer */
 
   /* In this example we want to open the input and output files before doing
@@ -459,12 +449,8 @@ do_read_JPEG_file(struct jpeg_decompress_struct *cinfo, char *infilename,
    */
 
   /* emit header for raw PPM format */
-#ifdef WITH_12BIT
   fprintf(outfile, "P6\n%d %d\n%d\n", WIDTH, HEIGHT,
           cinfo->data_precision == 12 ? MAXJ12SAMPLE : MAXJSAMPLE);
-#else
-  fprintf(outfile, "P6\n%d %d\n%d\n", WIDTH, HEIGHT, MAXJSAMPLE);
-#endif
 
   /* Step 4: set parameters for decompression */
 
@@ -488,12 +474,10 @@ do_read_JPEG_file(struct jpeg_decompress_struct *cinfo, char *infilename,
   /* Samples per row in output buffer */
   row_stride = cinfo->output_width * cinfo->output_components;
   /* Make a one-row-high sample array that will go away when done with image */
-#ifdef WITH_12BIT
   if (cinfo->data_precision == 12)
     buffer12 = (J12SAMPARRAY)(*cinfo->mem->alloc_sarray)
       ((j_common_ptr)cinfo, JPOOL_IMAGE, row_stride, 1);
   else
-#endif
     buffer = (*cinfo->mem->alloc_sarray)
       ((j_common_ptr)cinfo, JPOOL_IMAGE, row_stride, 1);
 
@@ -503,7 +487,6 @@ do_read_JPEG_file(struct jpeg_decompress_struct *cinfo, char *infilename,
   /* Here we use the library's state variable cinfo->output_scanline as the
    * loop counter, so that we don't have to keep track ourselves.
    */
-#ifdef WITH_12BIT
   if (cinfo->data_precision == 12) {
     while (cinfo->output_scanline < cinfo->output_height) {
       /* jpeg12_read_scanlines expects an array of pointers to scanlines.
@@ -517,9 +500,7 @@ do_read_JPEG_file(struct jpeg_decompress_struct *cinfo, char *infilename,
                            ((buffer12[0][col] >> 8) & 0xFF);
       fwrite(buffer12[0], 1, row_stride * sizeof(J12SAMPLE), outfile);
     }
-  } else
-#endif
-  {
+  } else {
     while (cinfo->output_scanline < cinfo->output_height) {
       /* jpeg_read_scanlines expects an array of pointers to scanlines.
        * Here the array is only one element long, but you could ask for
@@ -593,10 +574,8 @@ usage(const char *progname)
   fprintf(stderr, "       %s decompress inputfile[.jpg] outputfile[.ppm]\n",
           progname);
   fprintf(stderr, "Switches (names may be abbreviated):\n");
-#ifdef WITH_12BIT
   fprintf(stderr, "  -precision N   Create JPEG file with N-bit data precision\n");
   fprintf(stderr, "                 (N is 8 or 12; default is 8)\n");
-#endif
   fprintf(stderr, "  -quality N     Compression quality (0..100; 5-95 is most useful range,\n");
   fprintf(stderr, "                 default is 75)\n");
 
@@ -637,7 +616,6 @@ main(int argc, char **argv)
     }
     arg++;                      /* advance past switch marker character */
 
-#ifdef WITH_12BIT
     if (!strncasecmp(arg, "p", 1)) {
       /* Set data precision. */
       if (++argn >= argc)       /* advance to next argument */
@@ -645,9 +623,7 @@ main(int argc, char **argv)
       if (sscanf(argv[argn], "%d", &data_precision) < 1 ||
           (data_precision != 8 && data_precision != 12))
         usage(argv[0]);
-    } else
-#endif
-    if (!strncasecmp(arg, "q", 1)) {
+    } else if (!strncasecmp(arg, "q", 1)) {
       /* Quality rating (quantization table scaling factor). */
       if (++argn >= argc)       /* advance to next argument */
         usage(argv[0]);