From: Ilya Barygin <randomaction@ubuntu.com>
Subject: Fix warnings produced by -Werror=format-security

--- a/src/pngSave.cpp
+++ b/src/pngSave.cpp
@@ -55,8 +55,8 @@
 void trace(const char  *st, const char  *st2=NULL){
   static FILE *f=NULL;
   if (!f) f=fopen("trace.txt","wt");
-  fprintf(f,st);
-  if (st2) fprintf(f,st2);
+  fprintf(f,"%s",st);
+  if (st2) fprintf(f,"%s",st2);
   fflush(f);
 }