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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines