make SDL_qsort.c to actually link (missing SDL_stdinc.h include)

also sync its comments with version 1.15 from mainstream

src/stdlib/SDL_qsort.c

@@ -22,6 +22,7 @@
 #include "SDL_config.h"
 
 #ifndef HAVE_QSORT
+#include "SDL_stdinc.h"
 
 #ifdef assert
 #undef assert
@@ -50,7 +51,7 @@
 
 /*
 This code came from Gareth McCaughan, under the zlib license.
-Specifically this: https://www.mccaughan.org.uk/software/qsort.c-1.14
+Specifically this: https://www.mccaughan.org.uk/software/qsort.c-1.15
 
 Everything below this comment until the HAVE_QSORT #endif was from Gareth
 (any minor changes will be noted inline).
@@ -129,17 +130,21 @@ benefit!
  *   2016-02-21 v1.14 Replace licence with 2-clause BSD,
  *                    and clarify a couple of things in
  *                    comments. No code changes.
+ *   2016-03-10 v1.15 Fix bug kindly reported by Ryan Gordon
+ *                    (pre-insertion-sort messed up).
+ *                    Disable DEBUG_QSORT by default.
+ *                    Tweak comments very slightly.
  */
 
+/* BEGIN SDL CHANGE ... commented this out with an #if 0 block. --ryan. */
+#if 0
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 
-/* BEGIN SDL CHANGE ... commented this out with an #if 0 block. --ryan. */
-#if 0
-#define DEBUG_QSORT
+#undef DEBUG_QSORT
 
-static char _ID[]="<qsort.c gjm 1.14 2016-02-21>";
+static char _ID[]="<qsort.c gjm 1.15 2016-03-10>";
 #endif
 /* END SDL CHANGE ... commented this out with an #if 0 block. --ryan. */
 
@@ -302,7 +307,9 @@ typedef struct { char * first; char * last; } stack_entry;
  * We find the smallest element from the first |nmemb|,
  * or the first |limit|, whichever is smaller;
  * therefore we must have ensured that the globally smallest
- * element is in the first |limit|.
+ * element is in the first |limit| (because our
+ * quicksort recursion bottoms out only once we
+ * reach subarrays smaller than |limit|).
  */
 #define PreInsertion(swapper,limit,sz)		\
   first=base;					\
@@ -485,7 +492,7 @@ fprintf(stderr, "after partitioning first=#%lu last=#%lu\n", (first-(char*)base)
       Recurse(TRUNC_words)
     }
   }
-  PreInsertion(SWAP_words,(TRUNC_words/WORD_BYTES),WORD_BYTES);
+  PreInsertion(SWAP_words,TRUNC_words/WORD_BYTES,WORD_BYTES);
   /* Now do insertion sort. */
   last=((char*)base)+nmemb*WORD_BYTES;
   for (first=((char*)base)+WORD_BYTES;first!=last;first+=WORD_BYTES) {