feat: expose LIBXML_THREAD_ENABLED in version info

Because libxml2 2.11 changed how it does thread support detection and
we're about to monkey with it in a patch.

For context, see:

- https://gitlab.gnome.org/GNOME/libxml2/-/issues/427
- oracle/truffleruby#3031

ext/nokogiri/nokogiri.c

@@ -202,6 +202,12 @@ Init_nokogiri(void)
   rb_const_set(mNokogiri, rb_intern("LIBXSLT_COMPILED_VERSION"), NOKOGIRI_STR_NEW2(LIBXSLT_DOTTED_VERSION));
   rb_const_set(mNokogiri, rb_intern("LIBXSLT_LOADED_VERSION"), NOKOGIRI_STR_NEW2(xsltEngineVersion));
 
+#ifdef LIBXML_THREAD_ENABLED
+  rb_const_set(mNokogiri, rb_intern("LIBXML_THREAD_ENABLED"), Qtrue);
+#else
+  rb_const_set(mNokogiri, rb_intern("LIBXML_THREAD_ENABLED"), Qfalse);
+#endif
+
 #ifdef NOKOGIRI_PACKAGED_LIBRARIES
   rb_const_set(mNokogiri, rb_intern("PACKAGED_LIBRARIES"), Qtrue);
 #  ifdef NOKOGIRI_PRECOMPILED_LIBRARIES

lib/nokogiri/version/info.rb

@@ -143,6 +143,7 @@ def to_hash
               libxml["source"] = "system"
             end
             libxml["memory_management"] = Nokogiri::LIBXML_MEMORY_MANAGEMENT
+            libxml["thread_enabled"] = Nokogiri::LIBXML_THREAD_ENABLED
             libxml["iconv_enabled"] = libxml2_has_iconv?
             libxml["compiled"] = compiled_libxml_version.to_s
             libxml["loaded"] = loaded_libxml_version.to_s

test/test_version.rb

@@ -61,6 +61,9 @@ def test_version_info_for_libxml
     assert_equal(Nokogiri::LIBXML_COMPILED_VERSION, version_info["libxml"]["compiled"])
     assert_match(VERSION_MATCH, version_info["libxml"]["compiled"])
 
+    # i _think_ we always expect this to be true, regardless of system or packaged
+    assert(version_info["libxml"]["thread_enabled"])
+
     assert_match(VERSION_MATCH, version_info["libxml"]["loaded"])
     Nokogiri::LIBXML_LOADED_VERSION =~ /(\d)(\d{2})(\d{2})/
     major = Regexp.last_match(1).to_i