fix(cmake): declspecs usage for msvc (#5190)

CMakeLists.txt

@@ -18,10 +18,19 @@ else()
   include(${CMAKE_CURRENT_LIST_DIR}/env_support/cmake/custom.cmake)
 endif()
 
+#[[
+    unfortunately CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS does not work for global data. 
+    for global data we still need decl specs.
+    Check out the docs to learn more about the limitations of CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS
+    https://cmake.org/cmake/help/latest/prop_tgt/WINDOWS_EXPORT_ALL_SYMBOLS.html#prop_tgt:WINDOWS_EXPORT_ALL_SYMBOLS
+
+    For all compiled sources within the library (i.e. basically all lvgl files) we need to use dllexport.
+    For all compiled sources from outside the library (i.e. files which include lvgl headers) we need to use dllimport.
+    We can do this by using CMakes INTERFACE and PRIVATE keyword.
+  ]]
 if (MSVC)
-  if (BUILD_SHARED_LIBS)
-    target_compile_definitions(lvgl PUBLIC LV_ATTRIBUTE_EXTERN_DATA=__declspec\(dllimport\))
-  else()
-    target_compile_definitions(lvgl PUBLIC LV_ATTRIBUTE_EXTERN_DATA=__declspec\(dllexport\))
-  endif()
+  target_compile_definitions(lvgl
+    INTERFACE LV_ATTRIBUTE_EXTERN_DATA=__declspec\(dllimport\)
+    PRIVATE LV_ATTRIBUTE_EXTERN_DATA=__declspec\(dllexport\)
+  )
 endif()