databases/mongodb50: turn off LTO
After upgrading snappy 1.1.0 -> 1.2.0, mongodb50 fails to start:
ld-elf.so.1: /usr/local/bin/mongod: Undefined symbol "_ZN6snappy8CompressEPKcmPNSt3__112basic_stringIcNS2_11char_traitsIcEENS2_9allocatorIcEEEE"
Apparentally turning off LTO fixes this.
PR: 278254