Install GNU Readline on Darwin if the readline is broken (refs #23)

The `has_broken_mac_readline` checks the existence of
`readline/rhconf.h`. This is the same mechanism as setup.py of CPython.
fix `has_broken_mac_openssl` for building Python.

`--with-*-dir` is only effective for Ruby's mkmf.rb.
enable `require_gcc` to confirm if there is $CC
use environment variable "${CPP}" if defined
Merge branch 'has_broken_mac_readline' (fixes #23)