[Backport #21187]

This reverts commit a495e6a44ce8cff17461b250e32ab63e409a642d.

This break extension builds:

```
/Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'String#replace': can't modify frozen String: "$(SDKROOT)$(prefix)/include" (FrozenError)
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:321:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:314:in 'block in RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'String#gsub'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:307:in 'RbConfig.expand'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:325:in 'block in <module:RbConfig>'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in 'Hash#each_value'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:324:in '<module:RbConfig>'
        from /Users/hsbt/Documents/github.com/ruby/ruby/rbconfig.rb:11:in '<top (required)>'
        from ./ext/extmk.rb:42:in 'Kernel#require'
        from ./ext/extmk.rb:42:in '<main>'
make[1]: *** [ext/configure-ext.mk:70: ext/json/exts.mk] Error 1
```

The purpose of this commit is to fix Bug #21188.  We need to detect when
stdin has run in to an EOF case.  Unfortunately we can't _call_ the eof
function on IO because it will block.

Here is a short script to demonstrate the issue:

```ruby
x = STDIN.gets
puts x
puts x.eof?
```

If you run the script, then type some characters (but _NOT_ a newline),
then hit Ctrl-D twice, it will print the input string.  Unfortunately,
calling `eof?` will try to read from STDIN again causing us to need a
3rd Ctrl-D to exit the program.

Before introducing the EOF callback to Prism, the input loop looked
kind of like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if str.nil?
    p :DONE
  end
end
```

Which required 3 Ctrl-D to exit.  If we naively changed it to something
like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if STDIN.eof?
    p :DONE
  end
end
```

It would still require 3 Ctrl-D because `eof?` would block.  In this
patch, we're wrapping the IO object, checking the buffer for a newline
and length, and then using that to simulate a non-blocking eof? method.

This commit wraps STDIN and emulates a non-blocking `eof` function.

[Backport #21188]

* [ruby/prism] Accept a newline after the defined? keyword

[Bug #21197]

https://github.com/ruby/prism/commit/22be955ce9

* Fix a compilation error

Co-authored-by: Stan Lo <stan001212@gmail.com>

---------

Co-authored-by: Kevin Newton <kddnewton@gmail.com>
Co-authored-by: Stan Lo <stan001212@gmail.com>

This reverts commit e630a0f7ae909dafe2e1dbc47baf90cd795b4a5f.

	[ruby/prism] Freeze `Prism::VERSION`

	Closes https://github.com/ruby/prism/pull/3422

	https://github.com/ruby/prism/commit/b488a84253

	[ruby/prism] Accept a newline after the defined? keyword

	[Bug #21197]

	https://github.com/ruby/prism/commit/22be955ce9

[Bug #21161]

The `tolower` function provided by the libc is locale dependent
and can behave in ways you wouldn't expect for some value
of `LC_CTYPE`.

https://github.com/ruby/prism/commit/e3488256b4

Co-Authored-By: Nobuyoshi Nakada <nobu@ruby-lang.org>