LTO and harden switches

[revolting_peasant]

Three questions:

1. Is it useful to enable link-time optimizations (LTO) when building?
2. Is it _safe_ to enable LTO when building?
3. Why is executable "hardening" enabled when building? Why do I need "hardened" executables?

I'm building on a Devuan GNU/Linux Chimaera machine, GCC 10.2.1

[Pentarctagon]

For (1): in theory it should give some degree of performance improvement, though nobody has measured what it ends up being for Wesnoth. It does also result in a smaller executable file. Linking with LTO does also take significantly longer.

For (2): Wesnoth has a CI job that builds with LTO enabled that is able to pass our automated tests, so it should be fine.

For (3): It enables additional flags to improve security generally. For example adding -fPIE (position independent executable) so it can work in ASLR.