cross platform C99 library to get cpu features

 Cross-platform C library to retrieve CPU features(such as available
 instructions) at runtime.
 .
 Design rationale:
 .
  * Simple to use.
  * Extensible. Easy to add missing features or architectures.
  * Compatible with old compilers and available on many architectures so it
    can be used widely. It's implemented it in C99.
  * Sandbox-compatible. The library uses a variety of strategies to cope with
    sandboxed environments or when cpuid is unavailable.
  * Thread safe, no memory allocation, and raises no exceptions. cpu_features
    is suitable for implementing fundamental libc functions like malloc,
    memcpy, and memcmp.
  * Unit tested.