[libpng16] Reverted use png_get_libpng_ver(NULL) instead of PNG_LIBPNG_VER_STRING

in the manual, example.c, pngtest.c, and applications in the contrib directory.
It was incorrect advice.
diff --git a/example.c b/example.c
index aeb3735..f8067b9 100644
--- a/example.c
+++ b/example.c
@@ -294,7 +294,7 @@
     * the compiler header file version, so that we know if the application
     * was compiled with a compatible version of the library.  REQUIRED
     */
-   png_ptr = png_create_read_struct(png_get_libpng_ver(NULL),
+   png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
       png_voidp user_error_ptr, user_error_fn, user_warning_fn);
 
    if (png_ptr == NULL)
@@ -613,7 +613,7 @@
     * the library version is compatible in case we are using dynamically
     * linked libraries.
     */
-   *png_ptr = png_create_read_struct(png_get_libpng_ver(NULL),
+   *png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
        png_voidp user_error_ptr, user_error_fn, user_warning_fn);
 
    if (*png_ptr == NULL)
@@ -779,7 +779,7 @@
     * the library version is compatible with the one used at compile time,
     * in case we are using dynamically linked libraries.  REQUIRED.
     */
-   png_ptr = png_create_write_struct(png_get_libpng_ver(NULL),
+   png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
       png_voidp user_error_ptr, user_error_fn, user_warning_fn);
 
    if (png_ptr == NULL)