Adds the --enable-static configuration flag that is required in
the case of static builds. Removes explicit mentions of glibc,
and uses generic stdenv.cc.libc. The static attribute of the
libc is passed only when it exists.

Motivation for this change

Dropbear currently doesn't build with musl & AArch64. It is
affected by this bug #97351,
but even bypassing that, the resulting build segfaults. I don't
know the exact reason for this, but I think that building with musl,
and passing only a partial set of the flags required for a working
static build, the end result not working was not a huge wonder.
I tested this change to work correctly on AArch64 and x86_64,
both with musl and glibc static build.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.