Regex class: fix build error and, modify default enable.

* fix build error.
* implement method
  - ::new
  - #match
* add enc/ascii.c file from oniguruma-5.9.2
* but Regex literal doesn't yet support.

Sample code:
```ruby
puts "### Sample.1 ###"
str = "this is regexp"
rp = Regexp.new("^this is regexp")
p str == str # => true
p rp.match(str) # => #<MatchData "this is regexp">
p Regexp.last_match[0] #=> "this is regexp"

puts ""
puts "### Sample.2 ###"
str = "THIS IS REGEXP"
t1 = Regexp.compile("^this is regexp", Regexp::IGNORECASE)
t = t1.match(str)
puts t # => THIS IS REGEXP
```