install on Mac OS, libstdc++ deprecated

it works !!
try:
CPPFLAGS="-I/usr/local/include -L/usr/local/lib -stdlib=libc++ " pip install python-snappy

Start by reproducing the reported `pip install python-snappy` command on macOS with the shown CPPFLAGS and check how the installation handles the deprecated libstdc++. Review the installation/build entry points exposed by that command; done means the package installs successfully without relying on the reported workaround.

索引モデルが issue の本文から書いたものです。