Add visibility-hidden

This should make all symbols that are not marked otherwise have hidden
visibility. There still may be exposed symbols if marked with respective
attributes.

(cherry picked from commit d18fa929fbbf6d886e9122e1de948da94c29f54f)

CMakeLists.txt

@@ -35,6 +35,10 @@
 # Generate the buildfiles in a sub directory to not clutter the root directory with build files:
 # mkdir build && cd build && cmake -G "Unix Makefiles" .. && cmake --build .
 #
+# Ensure that you avoid exposing godot-cpp symbols - this might lead to hard to debug errors if you ever load multiple
+# plugins using difference godot-cpp versions. Use visibility hidden whenever possible:
+# set_target_properties(<all-my-plugin-related-targets> PROPERTIES CXX_VISIBILITY_PRESET hidden)
+#
 # Todo
 # Test build for Windows, Mac and mingw.
 
@@ -222,6 +226,7 @@ set_target_properties(${PROJECT_NAME}
 	PROPERTIES
 		CXX_EXTENSIONS OFF
 		POSITION_INDEPENDENT_CODE ON
+		CXX_VISIBILITY_PRESET hidden
 		ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin"
 		LIBRARY_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin"
 		RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin"