deps: define BUILDING_NGHTTP2 during nghttp2 build

Define BUILDING_NGHTTP2 in order that NGHTTP2_EXTERN is properly defined
when building the nghttp2 static library.

Move NGHTTP2_STATICLIB out of node.gyp because it is a property of the
nghttp2 static library, not the node executable.

PR-URL: https://github.com/nodejs/node/pull/15487


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>