[libpng16] Correct configure builds where build and source directories are

separate.  The include path of 'config.h' was erroneously made relative in
pngvalid.c in libpng 1.5.7.

contrib/libtests/config.h

@@ -1,17 +0,0 @@
-
-/*-
- * config.h
- *
- * Copyright (c) 2011 John Cunningham Bowler
- *
- * Last changed in libpng 1.6.0 [(PENDING RELEASE)]
- *
- * This code is released under the libpng license.
- * For conditions of distribution and use, see the disclaimer
- * and license in png.h
- *
- * Use the "config.h" generated by configure for libpng.  For indpendent
- * compilation of the .c files in this directory either make an appropriate
- * configure script to generate config.h or do not define HAVE_CONFIG_H.
- */
-#include "../../bsconfig.h"

contrib/libtests/pngstest.c

@@ -22,11 +22,18 @@
 #include <ctype.h>
 #include <math.h>
 
-#ifdef HAVE_CONFIG_H
-#  include "config.h"
+#if (defined HAVE_CONFIG_H) && !(defined PNG_NO_CONFIG_H)
+#  include <config.h>
 #endif
 
-#include "../../png.h"
+/* Define the following to use this test against your installed libpng, rather
+ * than the one being built here:
+ */
+#ifdef PNG_FREESTANDING_TESTS
+#  include <png.h>
+#else
+#  include "../../png.h"
+#endif
 
 #include "../tools/sRGB.h"
 

contrib/libtests/pngvalid.c

@@ -25,13 +25,22 @@
 
 #include <signal.h>
 
-#include "config.h"
+#if (defined HAVE_CONFIG_H) && !(defined PNG_NO_CONFIG_H)
+#  include <config.h>
+#endif
 
-#ifdef HAVE_FEENABLEEXCEPT
+#ifdef HAVE_FEENABLEEXCEPT /* from config.h, if included */
 #  include <fenv.h>
 #endif
 
-#include "../../png.h"
+/* Define the following to use this test against your installed libpng, rather
+ * than the one being built here:
+ */
+#ifdef PNG_FREESTANDING_TESTS
+#  include <png.h>
+#else
+#  include "../../png.h"
+#endif
 
 #if PNG_LIBPNG_VER < 10500
 /* This deliberately lacks the PNG_CONST. */

contrib/libtests/timepng.c

@@ -22,11 +22,18 @@
 
 #include <time.h>
 
-#ifdef HAVE_CONFIG_H
-#  include "config.h"
+#if (defined HAVE_CONFIG_H) && !(defined PNG_NO_CONFIG_H)
+#  include <config.h>
 #endif
 
-#include "../../png.h"
+/* Define the following to use this test against your installed libpng, rather
+ * than the one being built here:
+ */
+#ifdef PNG_FREESTANDING_TESTS
+#  include <png.h>
+#else
+#  include "../../png.h"
+#endif
 
 static int read_png(FILE *fp)
 {