Enable a way to globally enable/disable COZ tracepoints

It can be useful to leave coz tracepoints committed in the code but only
conditionally enable coz.

include/coz.h

@@ -8,6 +8,14 @@
 #if !defined(COZ_H)
 #define COZ_H
 
+// Embedders can define COZ_ENABLED explicitly to 1 or 0 to globally to enable/disable coz
+// tracepoints.
+#ifndef COZ_ENABLED
+#define COZ_ENABLED 1
+#endif
+
+#if COZ_ENABLED
+
 #ifndef __USE_GNU
 #  define __USE_GNU
 #endif
@@ -119,4 +127,17 @@ static void _call_coz_wake_other() {
 }
 #endif
 
+#else
+
+#define _COZ_NOOP() do {} while(0)
+#define COZ_PROGRESS_NAMED(name) _COZ_NOOP()
+#define COZ_PROGRESS() _COZ_NOOP()
+#define COZ_BEGIN(name) _COZ_NOOP()
+#define COZ_END(name) _COZ_NOOP()
+#define COZ_PRE_BLOCK() _COZ_NOOP()
+#define COZ_POST_BLOCK() _COZ_NOOP()
+#define COZ_WAKE_OTHER() _COZ_NOOP()
+
+#endif
+
 #endif