Clean up some #ifdefs.

Only the Mac doesn't have POSIX clocks. (And it still doesn't, a decade
later.)

glibc gained pthread_setname_np in 2.12.

Only the Mac doesn't have prctl.

Change-Id: I218e409f7e133736e15fb68e8a254cdc5799d667
diff --git a/runtime/globals.h b/runtime/globals.h
index e531c3a..93026da 100644
--- a/runtime/globals.h
+++ b/runtime/globals.h
@@ -108,7 +108,7 @@
   kTraceClockSourceDual,  // Both wall and thread CPU clocks.
 };
 
-#if defined(HAVE_POSIX_CLOCKS)
+#if defined(__linux__)
 static constexpr TraceClockSource kDefaultTraceClockSource = kTraceClockSourceDual;
 #else
 static constexpr TraceClockSource kDefaultTraceClockSource = kTraceClockSourceWall;