Summary: Currently open source rocksdb only builds with tcmalloc. This diff first checks if jemalloc is available. If it is, it compiles with jemalloc. If it isn't, it checks for tcmalloc.
Test Plan: Tried this out on my Ubuntu virtual machine and confirms that jemalloc is correctly detected and compiled.
Reviewers: MarkCallaghan, yhchiang, rven, sdong
Reviewed By: sdong
Subscribers: adamretter, meyering, dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D36789
fd7a357318