[Rpm-maint] [rpm-software-management/rpm] Use -flto=auto as a default for _lto_cflags. (#809)

[marxin]

> I can see -flto=auto being useful for the average upstream makefile defaults but I'm not convinced this is the right thing to do in rpm context: we'd want our parallel activities controlled via the same central tunables ($RPM_BUILD_NCPUS / %{_smp_ncpus_max}).

Which is way you want to controll a build system (make). Note that `-flto=N` is used for parallel linking phase and with the new option `-flto=auto` we can respect make's jobserver parallelism level
and communication with it. And as a fallback we want to make linking as parallel as possible. That's what we do in openSUSE right now.

> If LTO falls back to detecting number of cores on its own, that goal was missed.
> 
> Is there a summary of the gcc community reasoning someplace or can you provide one?

The main problem is that we see a lot of differences due to package builds with different `-flto=N` values. That's because some packages embed options (for `--help`) option. And mainly we want to have a bitwise identical `rpm` for reproducible builds. That's why we want to leave `-flto=N` option.

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