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.

djpeg.c

@@ -660,11 +660,9 @@ main(int argc, char **argv)
 #endif
 #ifdef GIF_SUPPORTED
   case FMT_GIF:
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12)
       dest_mgr = j12init_write_gif(&cinfo, TRUE);
     else
-#endif
       dest_mgr = jinit_write_gif(&cinfo, TRUE);
     break;
   case FMT_GIF0:
@@ -673,11 +671,9 @@ main(int argc, char **argv)
 #endif
 #ifdef PPM_SUPPORTED
   case FMT_PPM:
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12)
       dest_mgr = j12init_write_ppm(&cinfo);
     else
-#endif
       dest_mgr = jinit_write_ppm(&cinfo);
     break;
 #endif
@@ -717,7 +713,6 @@ main(int argc, char **argv)
     (*dest_mgr->start_output) (&cinfo, dest_mgr);
     cinfo.output_height = tmp;
 
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12) {
       /* Process data */
       while (cinfo.output_scanline < skip_start) {
@@ -736,9 +731,7 @@ main(int argc, char **argv)
                                               dest_mgr->buffer_height);
         (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
       }
-    } else
-#endif
-    {
+    } else {
       /* Process data */
       while (cinfo.output_scanline < skip_start) {
         num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,
@@ -772,11 +765,9 @@ main(int argc, char **argv)
       exit(EXIT_FAILURE);
     }
 
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12)
       jpeg12_crop_scanline(&cinfo, &crop_x, &crop_width);
     else
-#endif
       jpeg_crop_scanline(&cinfo, &crop_x, &crop_width);
     if (dest_mgr->calc_buffer_dimensions)
       (*dest_mgr->calc_buffer_dimensions) (&cinfo, dest_mgr);
@@ -791,7 +782,6 @@ main(int argc, char **argv)
     (*dest_mgr->start_output) (&cinfo, dest_mgr);
     cinfo.output_height = tmp;
 
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12) {
       /* Process data */
       if ((tmp = jpeg12_skip_scanlines(&cinfo, crop_y)) != crop_y) {
@@ -812,9 +802,7 @@ main(int argc, char **argv)
                 progname, tmp, cinfo.output_height - crop_y - crop_height);
         exit(EXIT_FAILURE);
       }
-    } else
-#endif
-    {
+    } else {
       /* Process data */
       if ((tmp = jpeg_skip_scanlines(&cinfo, crop_y)) != crop_y) {
         fprintf(stderr, "%s: jpeg_skip_scanlines() returned %u rather than %u\n",
@@ -841,7 +829,6 @@ main(int argc, char **argv)
     /* Write output file header */
     (*dest_mgr->start_output) (&cinfo, dest_mgr);
 
-#ifdef WITH_12BIT
     if (cinfo.data_precision == 12) {
       /* Process data */
       while (cinfo.output_scanline < cinfo.output_height) {
@@ -849,9 +836,7 @@ main(int argc, char **argv)
                                               dest_mgr->buffer_height);
         (*dest_mgr->put_pixel_rows) (&cinfo, dest_mgr, num_scanlines);
       }
-    } else
-#endif
-    {
+    } else {
       /* Process data */
       while (cinfo.output_scanline < cinfo.output_height) {
         num_scanlines = jpeg_read_scanlines(&cinfo, dest_mgr->buffer,