Default to --disable-static on TruffleRuby

* Shared libraries are more flexible and compile faster, see
  https://github.com/Shopify/oracle-truffleruby-collab/issues/11
* Static libraries can still be chosen (e.g., for testing) with:
  gem install nokogiri -- --use-system-libraries=false --enable-static

ext/nokogiri/extconf.rb

@@ -614,7 +614,7 @@ def do_clean
 else
   message "Building nokogiri using packaged libraries.\n"
 
-  static_p = enable_config("static", true)
+  static_p = enable_config("static", RUBY_ENGINE != 'truffleruby')
   message "Static linking is #{static_p ? 'enabled' : 'disabled'}.\n"
 
   cross_build_p = enable_config("cross-build")