3442865360 build: Use Link Time Optimization for Qt code on Linux (Hennadii Stepanov)
ebce66e532 build: pass -fno-lto when building expat (fanquake)
Pull request description:
See: https://www.qt.io/blog/2019/01/02/qt-applications-lto
`bitcon-qt` unstripped size:
| host | master (31c6309cc6) | this PR, depends built with `LTO=1` |
|---|:-:|:-:|
| x86_64-pc-linux-gnu | 42 MB | 35 MB |
| arm-linux-gnueabihf | 31 MB | 26 MB |
| aarch64-linux-gnu | 41 MB | 32 MB |
| powerpc64-linux-gnu | 51 MB | 41 MB |
| powerpc64le-linux-gnu | 48 MB | 39 MB |
| riscv64-linux-gnu | 35 MB | 29 MB |
Based on the first commit from bitcoin/bitcoin#25391.
Using LTO for macOS and Windows hosts has some issues which could be addressed in follow ups.
x86_64 build:
![image](https://user-images.githubusercontent.com/32963518/179326902-f91853ca-23c1-4c04-9a6d-161b695f27b5.png)
ACKs for top commit:
fanquake:
ACK 3442865360
Tree-SHA512: 03eef2568358df9336e24d6c4e12f28b89d649076fb74e7e5303d61e52865c2360c5345a4fb2b1e4bdfdae194f273fc27a5f67e6cf797ed01a154f3da9117247