cmake: use -flto=auto compiler flag when supported

[illwieckz]

Use -flto=auto compiler flag when supported, this silence this GCC warning:

lto-wrapper: warning: using serial compilation of # LTRANS jobs

This also greatly speeds-up the linkage time as it enables LTO multithreading in GCC (either by using Make jobserver if detected, either by detecting CPU cores).

[slipher]

LGTM. I tried on MSYS2 and it didn't break anything, although the parallelism didn't really appear to work. Running under Ninja, there were 6 processes but only one of them ever used the CPU at a time.

[DolceTriade]

🤷 If we want faster linking, we should just use mold or one of the new fancy linkers, but no harm so LGTM

[illwieckz]

I tested with:

clang-10
clang-11
clang-12
clang-13
clang-14
clang-15
clang-16
clang-17
clang-18
clang-19
clang-20
gcc-10
gcc-11
gcc-12
gcc-13
gcc-14
mingw
icx
aocc
icc
zig
apple-clang
freebsd-clang

Either the flag is used and there is no error, either the flag is not used because detected as unsupported. I haven't checked one by one how it ended, but in all cases this never failed neither printed obvious warnings.