libpll 0.3.1

Changes

• Checks for older versions of clang and gcc to use assembly instructions
  for cpu features detection
• Include guards for pll.h
• Correct updating of padded eigen-decomposition arrays for models with a
  number of states not being a power of two
• Changed to the usage of builtin functions for cpu features detection
• Check for x86intrin.h

Notes

1. The file libpll-0.3.1-linux-x86_64.tar.gz is a pre-compiled binary of libpll.
2. The file libpll-src-0.3.1.tar.gz contains the libpll source code and does not require autotools installed.
3. Files v0.3.1.tar.gz and 0.3.1.zip are generated by GitHub, and require autotools installed (as when cloning the repository).