sectransp: add support for HTTP/2 in gcc builds

sectransp: add support for HTTP/2 in gcc builds

Before this patch `--http2` did not work in gcc builds with Secure
Transport, because ALPN relied on a compiler supporting the
`HAVE_BUILTIN_AVAILABLE` aka `__builtin_available()` feature. This
is clang-specific and missing from gcc (as of gcc v14).

Add support for ALPN and HTTP/2 when this compiler feature is missing.

Also drop test exceptions from GHA/macos in CI.

Follow-up to 092f6815c808489f1cea3df8449e16dff2c35e6b
Ref: c349bd668c91f2484ae21c0f361ddf497143093c #14097 (issue 15.)
Ref: #4314

Closes #16581