Environment variable for Ruby configuration

[FranklinYu]

E.g., do the same as

$ ruby-install ruby 2.3.0 -- --enable-shared --enable-dtrace CFLAGS="-O3"

with

$ export RUBY_INSTALL_OPTS='--enable-shared --enable-dtrace CFLAGS="-O3"'
$ ruby-install ruby 2.3.0

This has advantage that a user may want global (user-specific) Ruby configurations, such as --foo-lib-path=/my/path/to/foo. It is --with-readline-dir=/usr/local/opt/readline in my case (on OS X), but I believe there may be more use cases.

[postmodern]

This might not work since ruby-install can install other rubies, and while CRuby is currently the only ruby which supports additional configuration opts, what happens when we accidentally pass global CRuby configuration opts to another ruby that also has a ./configure script that accepts options?