Prevent a spurious compiler warning.  All regression tests pass.

util/qprintf.c

diff --git a/util/qprintf.c b/util/qprintf.c
index 1cfea75..3f12781 100644
--- a/util/qprintf.c
+++ b/util/qprintf.c
@@ -35,7 +35,7 @@
 
 void
 PrintGifError(int ErrorCode) {
-    char *Err = GifErrorString(ErrorCode);
+    const char *Err = GifErrorString(ErrorCode);
 
     if (Err != NULL)
         fprintf(stderr, "GIF-LIB error: %s.\n", Err);