The dev-libs/leveldb-1.18-r1 ebuild has IUSE="tcmalloc" but sets USE_TCMALLOC=no in src_configure(). Thus, dev-libs/leveldb is never actually linked with tcmalloc. The dependency on dev-util/google-perftools should be removed, along with the "tcmalloc" USE flag. -- OR (alternatively) -- USE_TCMALLOC should be set according to the "tcmalloc" USE flag.
Fixed in new version 1.23, in commit: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=aad4b3e0fe8379f6fa34c7b48b4584590025760c