title: How to use NEON optimization for ARM in libpng 1.5.x, I received link errors
body: <p>I found optimization for NEON in pngrutil.c. But when I define a macro "PNG_ARM_NEON" and try to compile the sources in android NDK, I get these link errors:</p>
  
  <pre><code>/libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_up_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_sub4_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_avg4_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_paeth4_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_sub3_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_avg3_neon'
  /libpng/pngrutil.c:3730: undefined reference to `png_read_filter_row_paeth3_neon'
  </code></pre>
  
  <p>I found that these functions are declared in "pngpriv.h", but couldn't find the implementations in any source file.</p>
  
  <p>Is the "NEON Optimization" already available for use in current version of libpng or just   "TODO"?</p>