Implement a freeze: parser option

If set to true all parsed objects will be
immediately frozen, and strings will be
deduplicated if the Ruby implementation
allows it.

ext/json/ext/parser/extconf.rb

@@ -3,4 +3,30 @@
 
 have_func("rb_enc_raise", "ruby.h")
 
+# checking if String#-@ (str_uminus) dedupes... '
+begin
+  a = -(%w(t e s t).join)
+  b = -(%w(t e s t).join)
+  if a.equal?(b)
+    $CFLAGS << ' -DSTR_UMINUS_DEDUPE=1 '
+  else
+    $CFLAGS << ' -DSTR_UMINUS_DEDUPE=0 '
+  end
+rescue NoMethodError
+  $CFLAGS << ' -DSTR_UMINUS_DEDUPE=0 '
+end
+
+# checking if String#-@ (str_uminus) directly interns frozen strings... '
+begin
+  require 'objspace'
+  s = rand.to_s.freeze
+  if (-s).equal?(s) && ObjectSpace.dump(s).include?('"fstring":true')
+    $CFLAGS << ' -DSTR_UMINUS_DEDUPE_FROZEN=1 '
+  else
+    $CFLAGS << ' -DSTR_UMINUS_DEDUPE_FROZEN=0 '
+  end
+rescue NoMethodError
+  $CFLAGS << ' -DSTR_UMINUS_DEDUPE_FROZEN=0 '
+end
+
 create_makefile 'json/ext/parser'