use env variable to switch mem manager and fix cmake

Former-commit-id: fa4f031bdb0e8b5ab9955736182c2b9593a25679

cpp/CHANGELOG.md

@@ -11,6 +11,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-149 - Fixed searching only one index file issue in distributed mode
 - MS-153 - fix c_str error when connecting to MySQL
 - MS-157 - fix changelog
+- MS-190 - use env variable to switch mem manager and fix cmake
 
 ## Improvement
 - MS-156 - Add unittest for merge result functions