databases/mongodb60: turn off LTO by default

Got an error report that was fixed by turning this off.
ld-elf.so.1: /usr/local/bin/mongod: Undefined symbol "_ZN6snappy8CompressEPKcmPNSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEE"

PR: 279821