Consider making the build-time dependency on nghttp2 optional

Right now, the main branch needs nghttp2 to be available in order for BIND 9 to build at all. Since nghttp2 is only required for DNS-over-HTTPS (DoH) support - which we promised to backport to BIND 9.16 at some point - it looks slightly harsh to have a new, hard requirement on another library (even if it is a rather ubiquitous and not version-picky one), especially in light of a future BIND 9.16 backport.

We should probably discuss whether nghttp2 should be considered mandatory for:

• BIND 9.17+
• BIND 9.16¹

Any changes in this area should be "announced" in:

• README.md
• PLATFORMS.md
• release notes.

1. Not a possibility any more as it has been decided that DoH support will not be backported to BIND 9.16. ↩