Switch to using WAPP_PLATFORM_CPP instead of __cplusplus

tests/arena/test_arena.h

@@ -3,9 +3,9 @@
 
 #include "wapp.h"
 
-#ifdef __cplusplus
+#ifdef WAPP_PLATFORM_CPP
 BEGIN_C_LINKAGE
-#endif // __cplusplus
+#endif // WAPP_PLATFORM_CPP
 
 TestFuncResult test_arena_init(void);
 TestFuncResult test_arena_init_succeeds_when_reserving_very_large_size(void);
@@ -16,8 +16,8 @@ TestFuncResult test_arena_realloc_smaller_size(void);
 TestFuncResult test_arena_clear(void);
 TestFuncResult test_arena_destroy(void);
 
-#ifdef __cplusplus
+#ifdef WAPP_PLATFORM_CPP
 END_C_LINKAGE
-#endif // __cplusplus
+#endif // WAPP_PLATFORM_CPP
 
 #endif // !TEST_ARENA_H