82 Fix blocker macOS problems

There were two main problems

1. Dynamic linking on macOS requires some extra hoop-jumping:

   ```cmake
   if(APPLE)
     target_link_options(nain4-tests PRIVATE -undefined dynamic_lookup)
   endif()
   ```

2. There was a clash of libc++ versions between clang 16 and the
   dependencies pulled in by Qt and Geant4. This caused very mysterious
   segfaults before main was entered on pretty much any executable.

   We got around this by overriding the libc++ version used by clang 16
   on Darwin. (The Linux version had no such problems).

   An [upstream fix](NixOS/nixpkgs#234710) is
   in the works. Once that lands, we can remove our hack.

Now that all this has been fixed, GHA failures on macOS are no longer
allowed.

We need to bump the tag in the client-side fetch-content test, which
will continue to fail until we push the corresponding tag to this merge
commit.

.github/workflows/test.yml

@@ -16,13 +16,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04]
+        os: [ubuntu-22.04, macos-12]
         py: [311]
         allow-fail: [false]
-        include:
-          - os: macos-12
-            py: 311
-            allow-fail: true
 
     steps:
       - uses: actions/checkout@v3.5.3

client_side_tests/client_fetch_content/CMakeLists.txt

@@ -10,7 +10,7 @@ set(FETCHCONTENT_UPDATES_DISCONNECTED ON CACHE BOOL "Cache package contents to a
 FetchContent_Declare(
   Nain4
   GIT_REPOSITORY https://github.com/jacg/nain4.git
-  GIT_TAG        v0.1.8
+  GIT_TAG        v0.1.9
   # make sure that no other nain4 installation is used
   OVERRIDE_FIND_PACKAGE
   SOURCE_SUBDIR nain4

client_side_tests/test_fetch_content_recompile.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+set -e
 
 tmp_dir=$(mktemp -d -t nain4-recompile-XXXXXX)
 test_dir=$(dirname "$(readlink -f "$0")")

flake.nix

@@ -22,29 +22,51 @@
           enableRaytracerX11   = false;
         });
 
+        # Should be able to remove this, once https://github.com/NixOS/nixpkgs/issues/234710 is merged
+        clang_16 = if pkgs.stdenv.isDarwin
+          then pkgs.llvmPackages_16.clang.override rec {
+            libc = pkgs.darwin.Libsystem;
+            bintools = pkgs.bintools.override { inherit libc; };
+            inherit (pkgs.llvmPackages) libcxx;
+            extraPackages = [
+              pkgs.llvmPackages.libcxxabi
+              # Use the compiler-rt associated with clang, but use the libc++abi from the stdenv
+              # to avoid linking against two different versions (for the same reasons as above).
+              (pkgs.llvmPackages_16.compiler-rt.override {
+                inherit (pkgs.llvmPackages) libcxxabi;
+              })
+            ];
+          }
+          else pkgs.llvmPackages.clang;
+
+        my-packages = with pkgs; [
+          my-geant4
+          geant4.data.G4PhotonEvaporation
+          geant4.data.G4EMLOW
+          geant4.data.G4RadioactiveDecay
+          geant4.data.G4ENSDFSTATE
+          geant4.data.G4SAIDDATA
+          geant4.data.G4PARTICLEXS
+          geant4.data.G4NDL
+          clang_16
+          clang-tools
+          cmake
+          cmake-language-server
+          catch2_3
+          just
+          gnused # For hacking CMAKE_EXPORT stuff into CMakeLists.txt
+          mdbook
+        ] ++ lib.optionals stdenv.isDarwin [
+
+        ] ++ lib.optionals stdenv.isLinux [
+        ];
+
       in {
 
         devShell = pkgs.mkShell.override { stdenv = pkgs.clang_16.stdenv; } {
           name = "G4-examples-devenv";
 
-          packages = with pkgs; [
-            my-geant4
-            geant4.data.G4PhotonEvaporation
-            geant4.data.G4EMLOW
-            geant4.data.G4RadioactiveDecay
-            geant4.data.G4ENSDFSTATE
-            geant4.data.G4SAIDDATA
-            geant4.data.G4PARTICLEXS
-            geant4.data.G4NDL
-            clang_16
-            clang-tools
-            cmake
-            cmake-language-server
-            catch2_3
-            just
-            gnused # For hacking CMAKE_EXPORT stuff into CMakeLists.txt
-            mdbook
-          ];
+          packages = my-packages;
 
           G4_DIR = "${pkgs.geant4}";
           G4_EXAMPLES_DIR = "${pkgs.geant4}/share/Geant4-11.0.4/examples/";

nain4/CMakeLists.txt

@@ -45,6 +45,10 @@ FILE(GLOB HEADERS ${PROJECT_SOURCE_DIR}/*.hh)
 
 add_library(Nain4 SHARED ${SOURCES} ${HEADERS})
 
+if(APPLE)
+    target_link_options(Nain4 PRIVATE -undefined dynamic_lookup)
+endif()
+
 #set_target_properties(Nain4 PROPERTIES ENABLE_EXPORTS ON)
 
 #----------------------------------------------------------------------------
@@ -80,6 +84,10 @@ set(ALL_TEST_SOURCES
 # TODO including headers in add_executable apparently makes them show up in IDEs. Verify how?
 add_executable(nain4-tests ${ALL_TEST_SOURCES})
 
+if(APPLE)
+  target_link_options(nain4-tests PRIVATE -undefined dynamic_lookup)
+endif()
+
 
 target_link_libraries(
   nain4-tests