nix: add support to build with GCC

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

nix/matar-clang.nix

@@ -0,0 +1,25 @@
+{ ... }: {
+  perSystem = { pkgs, src, ... }:
+    let
+      llvm = pkgs.llvmPackages_16;
+      stdenv = llvm.libcxxStdenv;
+
+      libraries = with pkgs; [
+        ((pkgs.fmt.override {
+          inherit stdenv;
+          enableShared = false;
+        }).overrideAttrs (oa: {
+          cmakeFlags = oa.cmakeFlags ++ [ "-DFMT_TEST=off" ];
+        })).dev
+
+        (catch2_3.override { inherit stdenv; }).out
+      ];
+    in
+    {
+      packages.matar-clang = pkgs.callPackage ./build.nix { inherit src libraries stdenv; };
+      devShells.matar-clang = pkgs.callPackage ./shell.nix {
+        inherit libraries stdenv;
+        tools = with pkgs; [ clang-tools_16 ];
+      };
+    };
+}