Parameter-less macros can cause problems with headerfiles.

In this case <sys/thread.h> on Solaris 2.x.
t_flag is now gone. Use Pike_interpreter.trace_level.

Rev: src/apply_low.h:1.14
Rev: src/builtin.cmod:1.105
Rev: src/error.c:1.101
Rev: src/gc.c:1.192
Rev: src/interpret.c:1.287
Rev: src/interpret.h:1.122
Rev: src/interpret_functions.h:1.126
Rev: src/interpreter.h:1.81
Rev: src/main.c:1.162
Rev: src/mapping.c:1.158
Rev: src/multiset.c:1.65
Rev: src/opcodes.c:1.131
Rev: src/svalue.c:1.154
Rev: src/threads.c:1.196