Migrate from Qt5 to Qt6

Minimal version of Qt is now 6.

inc/lektor/internal/os.h

@@ -74,12 +74,12 @@ enum {
 typedef void (*function_ptr)(void);
 
 /* Attributes */
-#define noreturn             __attribute__((noreturn))
+// #define noreturn             __attribute__((noreturn))
 #define UNUSED               __attribute__((unused))
 #define UNUSED_FUNCTION      UNUSED
 #define CONSTRUCTOR_FUNCTION __attribute__((constructor)) static void
 #define DESTRUCTOR_FUNCTION  __attribute__((destructor)) static void
-#define EXIT_FUNCTION        noreturn void
+#define EXIT_FUNCTION        __attribute__((noreturn)) void
 #define PRIVATE_FUNCTION     UNUSED_FUNCTION static inline
 #define FALLTHROUGH          __attribute__((fallthrough))