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.

(cherry picked from commit 9220f0b1125626b100a823b801cec4d8362ec523)

PR: 278254