CMakeLists.txt: use visibility=hidden by default, so that unneeded libtownengine.so symbols are not leaked

CMakeLists.txt

@@ -305,6 +305,7 @@ link_deps(twn_third_parties)
 give_options(${TWN_TARGET})
 include_deps(${TWN_TARGET})
 link_deps(${TWN_TARGET})
+target_compile_options(${TWN_TARGET} PRIVATE -fvisibility=hidden)
 target_link_libraries(${TWN_TARGET} PUBLIC twn_third_parties)
 target_include_directories(${TWN_TARGET} PRIVATE ${TWN_ROOT_DIR}/src)
 

include/twn_engine_api.h

@@ -4,7 +4,7 @@
 #if defined(__WIN32)
 	#define TWN_API __declspec(dllexport)
 #else
-	#define TWN_API
+	#define TWN_API __attribute__((visibility("default")))
 #endif
 
 #endif