capnproto: build shared libs, debuginfo, etc.

This makes several improvements to the `capnproto` derivation:
- Build shared libs
- Build debuginfo
  See: https://git.lix.systems/lix-project/lix/issues/550
- Set the correct cmake build type
- Set the correct cxxflags to build the coroutine library
- Always build with clang
- Work around a broken static build
  See: avdv/scalals@a2de0ef

See: https://git.lix.systems/lix-project/lix/issues/527

This upstreams the fixes from https://gerrit.lix.systems/c/lix/+/2074

Author: 9999years

pkgs/by-name/ca/capnproto/package.nix

@@ -1,13 +1,34 @@
 {
+  binutils,
   lib,
-  stdenv,
+  clangStdenv,
   fetchFromGitHub,
   cmake,
   openssl,
   zlib,
 }:
 
-stdenv.mkDerivation rec {
+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 = clangStdenv.mkDerivation {
+    pname = "empty-libgcc_eh";
+    version = "0";
+    dontUnpack = true;
+    installPhase = ''
+      mkdir -p "$out"/lib
+      "${binutils}"/bin/ar r "$out"/lib/libgcc_eh.a
+    '';
+  };
+in
+
+# NOTE: This must be `clang` because `gcc` is known to miscompile or ICE while
+# compiling `capnproto` coroutines.
+#
+# See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102051
+# See: https://gerrit.lix.systems/c/lix/+/1874
+clangStdenv.mkDerivation rec {
   pname = "capnproto";
   version = "1.1.0";
 
@@ -23,8 +44,27 @@ stdenv.mkDerivation rec {
   propagatedBuildInputs = [
     openssl
     zlib
+  ] ++ lib.optional (clangStdenv.cc.isClang && clangStdenv.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
+  # right now for the couple MB of closure size:
+  # https://git.lix.systems/lix-project/lix/issues/551
+  # outputs = [ "bin" "dev" "out" ];
+
+  cmakeFlags = [
+    (lib.cmakeBool "BUILD_SHARED_LIBS" true)
+    # Take optimization flags from CXXFLAGS rather than cmake injecting them
+    (lib.cmakeFeature "CMAKE_BUILD_TYPE" "None")
   ];
 
+  env = {
+    # Required to build the coroutine library
+    CXXFLAGS = "-std=c++20";
+  };
+
+  separateDebugInfo = true;
+
   meta = with lib; {
     homepage = "https://capnproto.org/";
     description = "Cap'n Proto cerealization protocol";
@@ -35,6 +75,6 @@ stdenv.mkDerivation rec {
     '';
     license = licenses.mit;
     platforms = platforms.all;
-    maintainers = [ ];
+    maintainers = lib.teams.lix.members;
   };
 }