With removeUnknownConfigureFlags, it's impossible to express a package
that needs --enable-static, but will not accept --disable-shared,
without overriding the result of removeUnknownConfigureFlags _again_
in pkgs/top-level/static.nix.
It would be much better (and more in line with the rest of Nixpkgs) if
we encoded changes needed for static builds in package definitions
themselves, rather than in an ever-expanding list in static.nix. This
is especially true when doing it in static.nix is going to require
multiple overrides to express what could be expressed with stdenv
options.
So as a step in that direction, and to fix the problem described
above, here I replace removeUnknownConfigureFlags with a new stdenv
option, dontAddStaticConfigureFlags. With this mechanism, a package
that needs one but not both of the flags just needs to set
dontAddStaticConfigureFlags and then set up configureFlags manually
based on stdenv.hostPlatform.isStatic.
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