Security and legal fixes, Yamato CI

[kwi-dk]

• Apply Debian patch for CVE-2017-17969. (Should not matter in a Unity context, but for good measure...)
• Ensure GPL license compliance.
• Add support for building on Yamato, including Stevedore artifact uploads.
• Switch to clang -O3 -flto, giving a 4–15% speedup in benchmarks (95% confidence):

Linux:
android-ndk-linux-x86_64  (853 MB, 18k files)   t_user: 15%▾  t_wall: 15%▾
referenceassemblies_v46   (1.5 MB, 134 files)   t_user:  7%▾  t_wall:  6%▾  max_rss:  2%▴
tundra-linux-x64          (1.2 MB, 2 files)     t_user: 15%▾  t_wall: 14%▾  max_rss:  6%▴

macOS:
android-ndk-linux-x86_64  (853 MB, 18k files)   t_user:  7%▾  t_wall:  7%▾
referenceassemblies_v46   (1.5 MB, 134 files)   t_user:  4%▾  t_wall:  4%▾
tundra-linux-x64          (1.2 MB, 2 files)     t_user:  8%▾  t_wall:  7%▾  n_reclaim:  4%▾  max_rss:  4%▾