[libpng17] Added #ifdef's to contrib/examples programs so people don't try

to compile them without the minimum required support enabled
(suggested by Flavio Medeiros).

contrib/examples/pngpixel.c

@@ -27,6 +27,8 @@
  */
 #include "../../png.h"
 
+#if defined(PNG_READ_SUPPORTED) && defined(PNG_SEQUENTIAL_READ_SUPPORTED)
+
 /* Return component 'c' of pixel 'x' from the given row. */
 static unsigned int
 component(png_const_bytep row, png_uint_32 x, unsigned int c,
@@ -366,3 +368,4 @@ int main(int argc, const char **argv)
 
    return result;
 }
+#endif /* READ && SEQUENTIAL_READ */