Explicitly set PKG_CONFIG_PATH when `--with-openssl-dir` is used for older Rubies.
Otherwise, Ruby will attempt to link to the latest OpenSSL found by pkg-config,
which could be incompatible.
Using `--with-ext=+` only has the indended effect (compiling all extensions that would normally get compiled) on Ruby 2.5+. On older versions, it causes none of the default extensions to get compiled, resulting in a defunct installation.
- Now only generates a `configure` script if explicitly specified
- Add ability to add arbitrary files to the tarball
- Cache most common tarballs in fixtures directory to speed up tests
