[ruby/json] Add support for Solaris 10 which lacks strnlen()

Check for existence of strnlen() and use alternative code if it is missing.

ruby/json@48d4bbc3a0

Author: ngoto

ext/json/parser/extconf.rb

@@ -5,6 +5,7 @@
 have_func("rb_hash_new_capa", "ruby.h") # RUBY_VERSION >= 3.2
 have_func("rb_hash_bulk_insert", "ruby.h") # Missing on TruffleRuby
 have_func("rb_category_warn", "ruby.h") # Missing on TruffleRuby
+have_func("strnlen", "string.h") # Missing on Solaris 10
 
 append_cflags("-std=c99")