spec: Rely on default -flto values instead of using auto
According to the GCC docs[1], -flto=auto does:
> Use -flto=auto to use GNU make’s job server, if available, or
> otherwise fall back to autodetection of the number of CPU threads
> present in your system.
clang does not support -flto=auto (or -flto=jobserver for that matter),
so using this option breaks the build with non-gcc compilers. Pass just
-flto instead and rely on gcc doing the right thing.
[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html