Cut down on the use of ZLIB_LIBRARY/ZLIB_INCLUDE_DIR.

They're not listed in the documentation for the FindZLIB module,
so we shouldn't use them unless necessary.

modules/core/CMakeLists.txt

@@ -1,6 +1,6 @@
 set(the_description "The Core Functionality")
 ocv_add_module(core ${ZLIB_LIBRARIES} OPTIONAL opencv_cudev)
-ocv_module_include_directories(${ZLIB_INCLUDE_DIR})
+ocv_module_include_directories(${ZLIB_INCLUDE_DIRS})
 
 if (HAVE_WINRT)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /ZW /GS /Gm- /AI\"${WINDOWS_SDK_PATH}/References/CommonConfiguration/Neutral\" /AI\"${VISUAL_STUDIO_PATH}/vcpackages\"")

modules/highgui/CMakeLists.txt

@@ -10,7 +10,7 @@ ocv_add_module(highgui opencv_imgproc OPTIONAL opencv_androidcamera)
 ocv_clear_vars(GRFMT_LIBS)
 
 if(HAVE_PNG OR HAVE_TIFF OR HAVE_OPENEXR)
-  ocv_include_directories(${ZLIB_INCLUDE_DIR})
+  ocv_include_directories(${ZLIB_INCLUDE_DIRS})
   list(APPEND GRFMT_LIBS ${ZLIB_LIBRARIES})
 endif()