[M5+] swap toolchain to gcc15Stdenv + libstdc++

flake.nix

@@ -10,33 +10,20 @@
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs { inherit system; };
-        llvmPkgs = pkgs.llvmPackages;
-        libcxxPkgs = {
-          catch2_3 = pkgs.catch2_3.override {
-            stdenv = llvmPkgs.libcxxStdenv;
-          };
-        };
       in {
-        devShells.default = llvmPkgs.libcxxStdenv.mkDerivation {
+        devShells.default = pkgs.gcc15Stdenv.mkDerivation {
           name = "cargoxx-dev";
           version = "0.1.0";
           nativeBuildInputs = [
             pkgs.cmake
             pkgs.ninja
-            pkgs.clang-tools
             pkgs.git
             pkgs.pkg-config
           ];
           buildInputs = [
             pkgs.sqlite
             pkgs.reproc
-            libcxxPkgs.catch2_3
-          ];
-          env.NIX_CFLAGS_COMPILE = toString [
-            "-stdlib=libc++"
-            "-Wno-unused-command-line-argument"
-            "-B${pkgs.lib.getLib pkgs.libcxx}/lib"
-            "-isystem ${pkgs.lib.getDev pkgs.libcxx}/include/c++/v1"
+            pkgs.catch2_3
           ];
           hardeningDisable = [ "all" ];
         };