Add USE_HTTP2 define

[jsha]

This abstracts across the two HTTP/2 backends: nghttp2 and Hyper.

Add our own define for the "h2" ALPN protocol, so TLS backends can use
it without depending on a specific HTTP backend.

Fixes #6949.

[bagder]

I think CMakeLists.txt also needs a fix for this?

[jsha]

I'm afraid I don't know CMake. Can you point me in roughly the right direction? I know I need to set the USE_HTTP2 #define when either Hyper or nghttp2 are enabled, but I can't see how #defines are added in the CMake config.

[bagder]

An alternative approach that works better independently of build system would be to mimic the QUIC way:

curl/lib/curl_setup.h

Lines 804 to 806 in 7843fe0

	#if defined(USE_NGTCP2) || defined(USE_QUICHE)
	#define ENABLE_QUIC
	#endif

[bagder]

vtls/openssl.c:2717:24: error: incompatible pointer to integer conversion assigning to 'unsigned char' from 'const char [3]' [-Werror,-Wint-conversion]
      protocols[cur++] = ALPN_H2;

[bagder]

It looks like nss needs something similar?

vtls/nss.c:2112:24: error: incompatible pointer to integer conversion assigning to 'unsigned char' from 'const char [3]' [-Werror,-Wint-conversion]
      protocols[cur++] = ALPN_H2;
                       ^ ~~~~~~~

[Lionz01]

``

[bagder]

Thanks!