Fix socket configuration on OS X 10.10

OS X 10.10
ruby 2.1.4
Compiler: clang

```
./configure --prefix ~/.rbenv/versions/2.1.4 --disable-install-doc --without-gcc --with-out-ext=tk --with-opt-dir=/usr/local/opt/libyaml:/usr/local/opt/openssl:/usr/local/opt/readline
```

```
configuring socket
fixing apple's netinet6/in6.rb ...No such file or directory @ rb_sysopen - /usr/include/netinet6/in6.h
Failed to configure socket. It will not be installed.
```

```
ls /usr/include/netinet6/in6.h
gls: cannot access /usr/include/netinet6/in6.h: No such file or directory
```