feat(build): default to --disable-static on TruffleRuby

* Shared libraries are more flexible and compile faster, see
  sparklemotion#2191 (comment)
* 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>

CHANGELOG.md

@@ -20,6 +20,7 @@ Nokogiri follows [Semantic Versioning](https://semver.org/), please see the [REA
 * Reduce the number of object allocations needed when parsing an HTML::DocumentFragment. [[#2087](https://github.com/sparklemotion/nokogiri/issues/2087)] (Thanks, [@ashmaroli](https://github.com/ashmaroli)!)
 * [JRuby] Update the algorithm used to calculate `Node#line` to be wrong less-often. The underlying parser, Xerces, does not track line numbers, and so we've always used a hacky solution for this method. [[#1223](https://github.com/sparklemotion/nokogiri/issues/1223)]
 * Introduce `--enable-system-libraries` and `--disable-system-libraries` flags to `extconf.rb`. These flags provide the same functionality as `--use-system-libraries` and the `NOKOGIRI_USE_SYSTEM_LIBRARIES` environment variable, but are more idiomatic. [[#2193](https://github.com/sparklemotion/nokogiri/issues/2193)] (Thanks, [@eregon](https://github.com/eregon)!)
+* [TruffleRuby] `--disable-static` is used by default on TruffleRuby, when `--disable-system-libraries` is set, which is more flexible and compiles faster, see [#2191](https://github.com/sparklemotion/nokogiri/issues/2191#issuecomment-780724627). [[#2193](https://github.com/sparklemotion/nokogiri/issues/2193)] (Thanks, [@eregon](https://github.com/eregon)!)
 
 
 ## 1.11.1 / 2021-01-06

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