Move the declaration from gif_lib.h to gif_lib_private.h to solve
conflicts when some .c-file #includes both stdlib.h and gif_lib.h.
See also https://sourceforge.net/p/giflib/bugs/110/

diff -ur giflib-5.1.4.orig/lib/dgif_lib.c giflib-5.1.4/lib/dgif_lib.c
--- giflib-5.1.4.orig/lib/dgif_lib.c	2018-02-28 20:31:02.294682673 +0100
+++ giflib-5.1.4/lib/dgif_lib.c	2018-02-28 22:38:11.659126414 +0100
@@ -396,7 +396,7 @@
 
     if (GifFile->SavedImages) {
         SavedImage* new_saved_images =
-            (SavedImage *)reallocarray(GifFile->SavedImages,
+            (SavedImage *)giflib_private_reallocarray(GifFile->SavedImages,
                             (GifFile->ImageCount + 1), sizeof(SavedImage));
         if (new_saved_images == NULL) {
             GifFile->Error = D_GIF_ERR_NOT_ENOUGH_MEM;
@@ -1108,7 +1108,7 @@
               if (ImageSize > (SIZE_MAX / sizeof(GifPixelType))) {
                   return GIF_ERROR;
               }
-              sp->RasterBits = (unsigned char *)reallocarray(NULL, ImageSize,
+              sp->RasterBits = (unsigned char *)giflib_private_reallocarray(NULL, ImageSize,
                       sizeof(GifPixelType));
 
               if (sp->RasterBits == NULL) {
diff -ur giflib-5.1.4.orig/lib/gifalloc.c giflib-5.1.4/lib/gifalloc.c
--- giflib-5.1.4.orig/lib/gifalloc.c	2018-02-28 20:31:02.294682673 +0100
+++ giflib-5.1.4/lib/gifalloc.c	2018-02-28 22:38:11.657126423 +0100
@@ -8,7 +8,7 @@
 #include <stdio.h>
 #include <string.h>
 
-#include "gif_lib.h"
+#include "gif_lib_private.h"
 
 #define MAX(x, y)    (((x) > (y)) ? (x) : (y))
 
@@ -188,7 +188,7 @@
 
         /* perhaps we can shrink the map? */
         if (RoundUpTo < ColorUnion->ColorCount) {
-            GifColorType *new_map = (GifColorType *)reallocarray(Map,
+            GifColorType *new_map = (GifColorType *)giflib_private_reallocarray(Map,
                                  RoundUpTo, sizeof(GifColorType));
             if( new_map == NULL ) {
                 GifFreeMapObject(ColorUnion);
@@ -232,7 +232,7 @@
     if (*ExtensionBlocks == NULL)
         *ExtensionBlocks=(ExtensionBlock *)malloc(sizeof(ExtensionBlock));
     else {
-        ExtensionBlock* ep_new = (ExtensionBlock *)reallocarray
+        ExtensionBlock* ep_new = (ExtensionBlock *)giflib_private_reallocarray
 				 (*ExtensionBlocks, (*ExtensionBlockCount + 1),
                                       sizeof(ExtensionBlock));
         if( ep_new == NULL )
@@ -325,7 +325,7 @@
     if (GifFile->SavedImages == NULL)
         GifFile->SavedImages = (SavedImage *)malloc(sizeof(SavedImage));
     else
-        GifFile->SavedImages = (SavedImage *)reallocarray(GifFile->SavedImages,
+        GifFile->SavedImages = (SavedImage *)giflib_private_reallocarray(GifFile->SavedImages,
                                (GifFile->ImageCount + 1), sizeof(SavedImage));
 
     if (GifFile->SavedImages == NULL)
@@ -355,7 +355,7 @@
             }
 
             /* next, the raster */
-            sp->RasterBits = (unsigned char *)reallocarray(NULL,
+            sp->RasterBits = (unsigned char *)giflib_private_reallocarray(NULL,
                                                   (CopyFrom->ImageDesc.Height *
                                                   CopyFrom->ImageDesc.Width),
 						  sizeof(GifPixelType));
@@ -369,7 +369,7 @@
 
             /* finally, the extension blocks */
             if (sp->ExtensionBlocks != NULL) {
-                sp->ExtensionBlocks = (ExtensionBlock *)reallocarray(NULL,
+                sp->ExtensionBlocks = (ExtensionBlock *)giflib_private_reallocarray(NULL,
                                       CopyFrom->ExtensionBlockCount,
 				      sizeof(ExtensionBlock));
                 if (sp->ExtensionBlocks == NULL) {
diff -ur giflib-5.1.4.orig/lib/gif_lib.h giflib-5.1.4/lib/gif_lib.h
--- giflib-5.1.4.orig/lib/gif_lib.h	2018-02-28 20:31:02.294682673 +0100
+++ giflib-5.1.4/lib/gif_lib.h	2018-02-28 20:31:43.135716712 +0100
@@ -244,9 +244,6 @@
                                      GifPixelType ColorTransIn2[]);
 extern int GifBitSize(int n);
 
-extern void *
-reallocarray(void *optr, size_t nmemb, size_t size);
-
 /******************************************************************************
  Support for the in-core structures allocation (slurp mode).              
 ******************************************************************************/
diff -ur giflib-5.1.4.orig/lib/gif_lib_private.h giflib-5.1.4/lib/gif_lib_private.h
--- giflib-5.1.4.orig/lib/gif_lib_private.h	2018-02-28 20:31:02.294682673 +0100
+++ giflib-5.1.4/lib/gif_lib_private.h	2018-02-28 22:38:11.657126423 +0100
@@ -54,6 +54,9 @@
     bool gif89;
 } GifFilePrivateType;
 
+extern void *
+giflib_private_reallocarray(void *optr, size_t nmemb, size_t size);
+
 #endif /* _GIF_LIB_PRIVATE_H */
 
 /* end */
diff -ur giflib-5.1.4.orig/lib/openbsd-reallocarray.c giflib-5.1.4/lib/openbsd-reallocarray.c
--- giflib-5.1.4.orig/lib/openbsd-reallocarray.c	2018-02-28 20:31:02.295682659 +0100
+++ giflib-5.1.4/lib/openbsd-reallocarray.c	2018-02-28 22:38:11.656126428 +0100
@@ -27,7 +27,7 @@
 #define MUL_NO_OVERFLOW	((size_t)1 << (sizeof(size_t) * 4))
 
 void *
-reallocarray(void *optr, size_t nmemb, size_t size)
+giflib_private_reallocarray(void *optr, size_t nmemb, size_t size)
 {
 	if ((nmemb >= MUL_NO_OVERFLOW || size >= MUL_NO_OVERFLOW) &&
 	    nmemb > 0 && SIZE_MAX / nmemb < size) {
