fix: make static build work again

I copied the workaround from here:
https://github.com/avdv/scalals/commit/a2de0eff59cb4c9be6f8a33b2957e31727f87b89
Properly fixing the issue upstream will be much more difficult.

Closes: https://git.lix.systems/lix-project/lix/issues/527
Change-Id: I967d53fa9e80510b620df485af448f76bd9aa52a

misc/capnproto.nix

@@ -1,5 +1,6 @@
 # FIXME: upstream to nixpkgs (do NOT build with gcc due to gcc coroutine bugs)
 {
+  binutils,
   lib,
   stdenv,
   fetchFromGitHub,
@@ -8,6 +9,20 @@
   zlib,
 }:
 assert stdenv.cc.isClang;
+let
+  # HACK: work around https://github.com/NixOS/nixpkgs/issues/177129
+  # Though this is an issue between Clang and GCC,
+  # so it may not get fixed anytime soon...
+  empty-libgcc_eh = stdenv.mkDerivation {
+    pname = "empty-libgcc_eh";
+    version = "0";
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p "$out"/lib
+      "${binutils}"/bin/ar r "$out"/lib/libgcc_eh.a
+    '';
+  };
+in
 stdenv.mkDerivation rec {
   pname = "capnproto";
   version = "1.0.2";
@@ -24,7 +39,7 @@ stdenv.mkDerivation rec {
   propagatedBuildInputs = [
     openssl
     zlib
-  ];
+  ] ++ lib.optional (stdenv.cc.isClang && stdenv.targetPlatform.isStatic) empty-libgcc_eh;
 
   # FIXME: separate the binaries from the stuff that user systems actually use
   # This runs into a terrible UX issue in Lix and I just don't want to debug it