allow building luatex with old libpng
patch by Peter Breitenlohner
taken from TeX Live repository
---
 source/texk/web2c/luatexdir/ChangeLog        |    4 ++++
 source/texk/web2c/luatexdir/image/writepng.w |    4 ++++
 2 files changed, 8 insertions(+)

Index: luatex-0.70.0/source/texk/web2c/luatexdir/ChangeLog
===================================================================
--- luatex-0.70.0.orig/source/texk/web2c/luatexdir/ChangeLog	2011-05-15 15:56:45.000000000 +0900
+++ luatex-0.70.0/source/texk/web2c/luatexdir/ChangeLog	2011-05-15 16:00:06.000000000 +0900
@@ -1,3 +1,7 @@
+2011-05-12  Peter Breitenlohner  <peb@mppmu.mpg.de>
+
+       * image/writepng.w: #define PNG_FP_1 for libpng < 1.5.0.
+
 2011-04-16  Peter Breitenlohner  <peb@mppmu.mpg.de>
 
 	* utils/synctex{-common.h,.c,.h} (removed): Use copies from
Index: luatex-0.70.0/source/texk/web2c/luatexdir/image/writepng.w
===================================================================
--- luatex-0.70.0.orig/source/texk/web2c/luatexdir/image/writepng.w	2011-05-15 15:56:55.000000000 +0900
+++ luatex-0.70.0/source/texk/web2c/luatexdir/image/writepng.w	2011-05-15 15:59:02.000000000 +0900
@@ -520,6 +520,10 @@
     boolean png_copy = true;
     double gamma = 0.0;
     png_fixed_point int_file_gamma = 0;
+#ifndef PNG_FP_1
+   /* for libpng < 1.5.0 */
+#define PNG_FP_1    100000
+#endif
     int i;
     int palette_objnum = 0;
     png_structp png_p;
