lib: merge ENABLE_QUIC C macro into USE_HTTP3

[vszakats]

Before this patch lib/curl_setup.h defined these two macros right
next to each other, then the source code used them interchangeably.

After this patch, USE_HTTP3 guards all HTTP/3 / QUIC features.
(Like USE_HTTP2 does for HTTP/2.) ENABLE_QUIC is no longer used.

This patch doesn't change the way HTTP/3 is enabled via autotools
or CMake. Builders who enabled HTTP/3 manually by defining both of
these macros via CPPFLAGS can now delete -DENABLE_QUIC.

Closes #13352

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I could not spot the difference between the meaning of these two macros
in context of curl's HTTP/3 support. Let me know if I missed something.

[talregev]

There is a several ways to build http3 with curl.
Can you elaborate about each of one, if it will not effect them after your patch?
I am more interested if you can able to compile openssl + nghttp3 to have http3 feature inside curl after your patch?

[vszakats]

My patch doesn't change how these features are enabled / selected.

OpenSSL + nghttp3 can be enabled the same way as before:
CMake: -DCURL_USE_OPENSSL=ON -DOPENSSL_ROOT_DIR=<path> -DUSE_OPENSSL_QUIC=ON
autotools: --with-openssl=<path> --with-openssl-quic

(If someone was using ENABLE_QUIC + USE_HTTP3 as raw CPPFLAGS to enable HTTP/3, ENABLE_QUIC no longer needs to be passed after this patch.)

[talregev]

Thank you for clearing it up!