feat(build): 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

Co-authored-by: Benoit Daloze <eregontp@gmail.com>

ext/nokogiri/extconf.rb

@@ -154,7 +154,8 @@ def config_clean?
 end
 
 def config_static?
-  enable_config("static", true)
+  default_static = !truffle?
+  enable_config("static", default_static)
 end
 
 def config_cross_build?
@@ -191,6 +192,10 @@ def nix?
   !(windows? || solaris? || darwin?)
 end
 
+def truffle?
+  ::RUBY_ENGINE == 'truffleruby'
+end
+
 def concat_flags(*args)
   args.compact.join(" ")
 end