Update double-conversion to rev 8 via SR 770685
    
    https://build.opensuse.org/request/show/770685
    by user pgajdos + dimstar_suse
    - Use proper cmake calls
    - Do not pull in py2 that is EOL but use python3 for build
    
    - Update to version 3.1.5 jsc#ECO-1190 (forwarded request 770656 from scarabeus_iv)