From: Paul Zander <negril.nx+gentoo@gmail.com>

Gentoo installs libpng as libpng16, so adjust the detection.

diff --git a/cmake/OpenCVFindLibsGrfmt.cmake b/cmake/OpenCVFindLibsGrfmt.cmake
index 46b5108..0ff88ea 100644
--- a/cmake/OpenCVFindLibsGrfmt.cmake
+++ b/cmake/OpenCVFindLibsGrfmt.cmake
@@ -245,9 +245,9 @@ if(NOT HAVE_SPNG AND WITH_PNG)
     include(FindPNG)
     if(PNG_FOUND)
       include(CheckIncludeFile)
-      check_include_file("${PNG_PNG_INCLUDE_DIR}/libpng/png.h" HAVE_LIBPNG_PNG_H)
-      if(HAVE_LIBPNG_PNG_H)
-        ocv_parse_header("${PNG_PNG_INCLUDE_DIR}/libpng/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
+      check_include_file("${PNG_PNG_INCLUDE_DIR}/libpng16/png.h" HAVE_LIBPNG16_PNG_H)
+      if(HAVE_LIBPNG16_PNG_H)
+        ocv_parse_header("${PNG_PNG_INCLUDE_DIR}/libpng16/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
       else()
         ocv_parse_header("${PNG_PNG_INCLUDE_DIR}/png.h" PNG_VERSION_LINES PNG_LIBPNG_VER_MAJOR PNG_LIBPNG_VER_MINOR PNG_LIBPNG_VER_RELEASE)
       endif()
@@ -255,7 +255,7 @@ if(NOT HAVE_SPNG AND WITH_PNG)
   endif()

   if(NOT PNG_FOUND)
-    ocv_clear_vars(PNG_LIBRARY PNG_LIBRARIES PNG_INCLUDE_DIR PNG_PNG_INCLUDE_DIR HAVE_LIBPNG_PNG_H PNG_DEFINITIONS)
+    ocv_clear_vars(PNG_LIBRARY PNG_LIBRARIES PNG_INCLUDE_DIR PNG_PNG_INCLUDE_DIR HAVE_LIBPNG16_PNG_H PNG_DEFINITIONS)

     set(PNG_LIBRARY libpng CACHE INTERNAL "")
     set(PNG_LIBRARIES ${PNG_LIBRARY})
diff --git a/cmake/templates/cvconfig.h.in b/cmake/templates/cvconfig.h.in
index d6c7875..005e9b3 100644
--- a/cmake/templates/cvconfig.h.in
+++ b/cmake/templates/cvconfig.h.in
@@ -79,7 +79,7 @@
 #cmakedefine HAVE_JPEG

 /* libpng/png.h needs to be included */
-#cmakedefine HAVE_LIBPNG_PNG_H
+#cmakedefine HAVE_LIBPNG16_PNG_H

 /* GDCM DICOM codec */
 #cmakedefine HAVE_GDCM
diff --git a/modules/imgcodecs/src/grfmt_png.cpp b/modules/imgcodecs/src/grfmt_png.cpp
index 388a3fc..4dde181 100644
--- a/modules/imgcodecs/src/grfmt_png.cpp
+++ b/modules/imgcodecs/src/grfmt_png.cpp
@@ -58,8 +58,8 @@
 #  define _FILE_OFFSET_BITS 0
 #endif

-#ifdef HAVE_LIBPNG_PNG_H
-#include <libpng/png.h>
+#ifdef HAVE_LIBPNG16_PNG_H
+#include <libpng16/png.h>
 #else
 #include <png.h>
 #endif