demos/http3: Use `SSL_write_ex2()` together with `SSL_WRITE_FLAG_CONCLUDE`

demos/http3: Use `SSL_write_ex2()` together with `SSL_WRITE_FLAG_CONCLUDE`

These calls were introduced by PR #23343.

Change also does a minor tweak to Makefile so CFLAGS and LDFLAGS variables
from the environment are respected.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23602)