Add a ChangeLog entry.

Author: juj

ChangeLog.md

@@ -17,6 +17,8 @@ See docs/process.md for how version tagging works.
 
 Current Trunk
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+- Removed preamble variable ENVIRONMENT_IS_PTHREAD. Use the variable
+  ENVIRONMENT_IS_WORKER instead to detect code that is running in a pthread.
 - All ports now install their headers into a shared directory under
   `EM_CACHE`.  This should not really be a user visible change although one
   side effect is that once a give ports is built its headers are then