add lto flag and test

[bash99]

https://www.percona.com/live/17/sessions/optimizing-mysql-without-sql-or-touching-mycnf

Link-time optimizationsAlso known as Full Program Optimization) Supported by both modern GCC and Clang with -flto option

worry about end binary size and memory usage on compile

[bash99]

https://github.com/google/autofdo is useful, but is a little beyond this project scope

[bash99]

LTO is super slow, super memory hungry, and require much disk space in /tmp (30G is a safe bet).
Performance seems up from another 10~15% for 8.0.

As https://bugs.mysql.com/bug.php?id=67790, 8.0 has WITH_LTO cmake flag and fix some bugs.
Don't know 5.7 will build ok. just make another try.