[ruby/zlib] In Zlib::GzipReader#eof? check if we're actually at eof

	Only consider it eof if we read ahead and something fills the buf.
	If not, we may only have empty blocks and the footer.

	Fixes https://github.com/ruby/zlib/pull/56

	https://github.com/ruby/zlib/commit/437bea8003

https://github.com/ruby/zlib/commit/2561e122ac

We use the Cloudflare fork of zlib
(https://github.com/cloudflare/zlib), which we find gives improved
performance on AWS Graviton ARM instances. That fork does not define
crc32_z and alder32_z functions.

Until two days ago, Ruby's zlib gem worked fine, because cloudflare zlib
_also_ did not define z_size_t, which meant Ruby did not try and use
these functions.

Since https://github.com/cloudflare/zlib/commit/a3ba99596d6271224d39ef9d6853511f51821e05
however, cloudflare zlib _does_ define z_size_t (but NOT crc32_z or
alder32_z). The zlib gem would try and use these nonexistant
functions and not compile.

This patch fixes it by actually specifically detecting the functions
that the gem wants to call, rather than just the presence of the
z_size_t type.

https://github.com/ruby/zlib/commit/c96e8b9a57

(https://github.com/ruby/zlib/pull/55)

zstream_discard_input was encoding and character-aware when given input is user-provided, so this discards `len` chars instead of `len` bytes.

Also Zlib.gunzip explains in its rdoc that it is equivalent with the following code, but this doesn't fail for UTF-8 String.

```ruby
string = %w[1f8b0800c28000000003cb48cdc9c9070086a6103605000000].pack("H*").force_encoding('UTF-8')
sio = StringIO.new(string)
p gz.read #=> "hello"
gz&.close
p Zlib.gunzip(string) #=> Zlib::DataError
```

Reported and discovered by eagletmt at https://twitter.com/eagletmt/status/1689692467929694209

https://github.com/ruby/zlib/commit/c5e58bc62a

https://github.com/ruby/zlib/commit/f1ce5e3e8d

https://github.com/ruby/zlib/commit/1ce6625fff

https://github.com/ruby/zlib/commit/e1ead85113

Upper bits affect the result of `crc32` in zlib 1.2.12.

https://github.com/ruby/zlib/commit/9ab6d04af1