strnlen is a GNU extension

--- libpng/pngerror.c
+++ libpng/pngerror.c
@@ -23,6 +23,16 @@
 png_default_warning PNGARG((png_structp png_ptr,
   png_const_charp warning_message));
 
+#ifndef _GNU_SOURCE 
+size_t strnlen(const char *s, size_t limit)
+{
+   size_t len = 0;
+   while ((len < limit) && (*s++))
+     len++;
+   return len;
+}
+#endif
+
 /* This function is called whenever there is a fatal error.  This function
  * should not be changed.  If there is a need to handle errors differently,
  * you should supply a replacement error function and use png_set_error_fn()