Trac #11696: libpng-1.2.35.p5 (Leif Leonhardy, Jean-Pierre Flori, 19 February 2013)
 * #11696: Correctly build and install libpng.
 * Delete symlinks from libpng.* in $SAGE_LOCAL/lib on Darwin only.
 * Fix order of additions of flags and do not drop user 

PR for issues #11696, #11696