[libpng12] Quiet unused variable warnings from clang by porting PNG_UNUSED()

from libpng-1.4.6.

pngwrite.c

@@ -1586,8 +1586,8 @@ png_write_png(png_structp png_ptr, png_infop info_ptr,
    /* It is REQUIRED to call this to finish writing the rest of the file */
    png_write_end(png_ptr, info_ptr);
 
-   transforms = transforms; /* Quiet compiler warnings */
-   params = params;
+   PNG_UNUSED(transforms) /* Quiet compiler warnings */
+   PNG_UNUSED(params)
 }
 #endif
 #endif /* PNG_WRITE_SUPPORTED */