MJIT is enabled whenever an MJIT header is used

```
$ benchmark-driver -v --rbenv 'before --jit;after --jit' --repeat-count=4 --alternate --output=all benchmark.yml
before --jit: ruby 3.0.0dev (2020-12-22T05:16:39Z master 692af8e8f8) +JIT [x86_64-linux]
after --jit: ruby 3.0.0dev (2020-12-22T06:05:59Z master d2ca23fdbc) +JIT [x86_64-linux]
last_commit=MJIT is enabled whenever an MJIT header is used
Calculating -------------------------------------
                             before --jit           after --jit
Optcarrot 180 frames    86.61115328062763     91.49245232931852 fps
                        90.18653732453176     92.31139517827663
                        91.29817884736318     93.84437713805814
                        92.76118222412521     93.91929147901106
```

mjit.h

@@ -187,7 +187,11 @@ mjit_exec(rb_execution_context_t *ec)
 
 void mjit_child_after_fork(void);
 
+#  ifdef MJIT_HEADER
+#define mjit_enabled true
+#  else // MJIT_HEADER
 extern bool mjit_enabled;
+#  endif // MJIT_HEADER
 VALUE mjit_pause(bool wait_p);
 VALUE mjit_resume(void);
 void mjit_finish(bool close_handle_p);
@@ -207,7 +211,7 @@ static inline void mjit_remove_class_serial(rb_serial_t class_serial){}
 static inline VALUE mjit_exec(rb_execution_context_t *ec) { return Qundef; /* unreachable */ }
 static inline void mjit_child_after_fork(void){}
 
-#define mjit_enabled 0
+#define mjit_enabled false
 static inline VALUE mjit_pause(bool wait_p){ return Qnil; } // unreachable
 static inline VALUE mjit_resume(void){ return Qnil; } // unreachable
 static inline void mjit_finish(bool close_handle_p){}