Refine logic of parsing PNG version

Currently, if `PNG_FOUND`, cmake scripts will check include and parse
header while we can use `PNG_VERSION_STRING` conveniently. If
`BUILD_PNG`, parse version from `PNG_LIBPNG_VER_STRING` directly is more
convenient than parsing major, minor and patch and concatenate them.

The comment of png.h also supports this.
```
/* These should match the first 3 components of PNG_LIBPNG_VER_STRING: */
```
https://github.com/glennrp/libpng/blob/libpng16/png.h#L287

This patch also modifies `ocv_parse_header_version` macro to receive
another parameter to make it more general.

The reason why changing `PNG_VERSION` to `PNG_VERSION_STRING` is to be
consistent with cmake's FindPNG.

This patch removes `HAVE_LIBPNG_PNG_H` variable because `PNG_INCLUDE_DIR`
is where to find png.h, etc according to
https://cmake.org/cmake/help/latest/module/FindPNG.html.

This patch also removes `PNG_PNG_INCLUDE_DIR` variable which is an
advanced variable used in cmake's FindPNG and is not used in opencv.

modules/imgcodecs/src/grfmt_png.cpp

@@ -58,11 +58,7 @@
 #  define _FILE_OFFSET_BITS 0
 #endif
 
-#ifdef HAVE_LIBPNG_PNG_H
-#include <libpng/png.h>
-#else
 #include <png.h>
-#endif
 #include <zlib.h>
 
 #include "grfmt_png.hpp"