* Only from Ruby 2.4+ since older releases don't use --enable-shared by default.
* As with releases it can be disabled by setting RUBY_CONFIGURE_OPTS=--disable-shared.
This merges "standard_build" and "standard_install" build steps into one again
and modifies "standard_install_with_bundled_gems" to just invoke "standard"
with the addition of `update-gems` & `extract-gems` targets.