- Disable Intel SSE and SSE2 on i386, since Debian supports some i386
processors without those extensions. Keep them enabled on amd64, since all
amd64 processors have them.
- Disable Intel SSSE3 entirely, since no i386 processor supports it and Debian
supports amd64 processors without it.
- Disable NEON on armel and armhf, since no armel processor supports NEON and
Debian supports some armhf processors without it. Keep it enabled on arm64,
since all arm64 processors have it.
- To minimize the possibility of future ABI breakage, treat absl::any,
absl::optional, absl::string_view, and absl::variant as their own types
(rather than aliases for the std:: versions), and compile everything in an
inline namespace.