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