configure.ac: permit explicitly turning off brotli and zstd

• in the summary at the end print the wslay and nghttp2 status, even when --with-http was not passed to ./configure

• remove AC_MSG_NOTICEs. The summary at the end has the purpose to substitute these.

The rationale is that no http client (CUA) uses Zstandard currently.

Besides, when httpd runs behind a reverse proxy, the proxy server can take care of
the compression, which it does any way for other stuff, so this change makes httpd
lighter.