minor cmake update

cmake/OpenCVIOLibs.cmake

@@ -19,7 +19,7 @@ if(NOT ZLIB_FOUND)
   set(ZLIB_INCLUDE_DIR "${${ZLIB_LIBRARY}_SOURCE_DIR}" "${${ZLIB_LIBRARY}_BINARY_DIR}")
 endif()
 
-################### libtiff - optional
+################### libtiff - optional (should be searched after zlib)
 if(WITH_TIFF)
   if(BUILD_TIFF)
     set(TIFF_FOUND FALSE)
@@ -67,14 +67,14 @@ if(WITH_JASPER AND NOT JASPER_FOUND)
   set(JASPER_INCLUDE_DIR "${${JASPER_LIBRARY}_SOURCE_DIR}")
 endif()
 
-################### libpng - optional
+################### libpng - optional (should be searched after zlib)
 if(WITH_PNG)
   if(BUILD_PNG)
     set(PNG_FOUND FALSE)
   else()
     include(FindPNG)
     if(PNG_FOUND)
-      check_include_file(${PNG_PNG_INCLUDE_DIR}/png.h HAVE_PNG_H)
+      check_include_file(${PNG_PNG_INCLUDE_DIR}/png.h        HAVE_PNG_H)
       check_include_file(${PNG_PNG_INCLUDE_DIR}/libpng/png.h HAVE_LIBPNG_PNG_H)
     endif()
   endif()