build: Allow linking against an external copy of nghttp2.

The version of nghttp2 in deps/ does not build on CloudABI, even though
the official version does. Though this is an issue on its own that needs
to be resolved, it is currently a bit hard to work around this. There is
no switch to link against an external version of nghttp2, even though we
do provide this option for other libraries.

This change adds configure flags, similar to the ones we have for
OpenSSL, zlib, http_parser, libuv, etc. and makes the dependency on
deps/nghttp2 optional.

PR-URL: https://github.com/nodejs/node/pull/16788


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>