Only use exit(77) in configure builds

Signed-off-by: John Bowler <jbowler@acm.org>

contrib/libtests/pngimage.c

@@ -36,6 +36,15 @@
 #  include <setjmp.h> /* because png.h did *not* include this */
 #endif
 
+/* 1.6.1 added support for the configure test harness, which uses 77 to indicate
+ * a skipped test, in earlier versions we need to succeed on a skipped test, so:
+ */
+#if PNG_LIBPNG_VER >= 10601 && defined(HAVE_CONFIG_H)
+#  define SKIP 77
+#else
+#  define SKIP 0
+#endif
+
 #if defined(PNG_INFO_IMAGE_SUPPORTED) && defined(PNG_SEQUENTIAL_READ_SUPPORTED)\
     && (defined(PNG_READ_PNG_SUPPORTED) || PNG_LIBPNG_VER < 10700)
 /* If a transform is valid on both read and write this implies that if the
@@ -1682,6 +1691,6 @@ int
 main(void)
 {
    fprintf(stderr, "pngimage: no support for png_read/write_image\n");
-   return 77;
+   return SKIP;
 }
 #endif