simplify CMakeList with cmake scripts

build/cmake/enable_png.cmake

@@ -0,0 +1,17 @@
+option(NANA_CMAKE_ENABLE_PNG "Enable the use of PNG" OFF)
+option(NANA_CMAKE_LIBPNG_FROM_OS "Use libpng from operating system." ON)
+
+# todo: decide - PUBLIC vs PRIVATE
+
+if(NANA_CMAKE_ENABLE_PNG)
+    if(NANA_CMAKE_LIBPNG_FROM_OS)
+        find_package(PNG)
+        if(PNG_FOUND)
+            target_include_directories(nana PUBLIC ${PNG_INCLUDE_DIRS})
+            target_compile_options    (nana PUBLIC ${PNG_LIBRARIES})
+            target_compile_definitions(nana PUBLIC NANA_ENABLE_PNG USE_LIBPNG_FROM_OS)
+        endif()
+    else()
+        target_compile_definitions(nana PUBLIC NANA_ENABLE_PNG)
+    endif()
+endif()