update gcc debug compile option for coverage

.envrc

@@ -0,0 +1 @@
+use flake

CMakeLists.txt

@@ -10,7 +10,7 @@ cppm_cxx_standard(17)
 cppm_compiler_option(
    DEBUG
       CLANG "-fprofile-instr-generate -fcoverage-mapping"
-      GCC   "-fprofile-arcs -ftest-coverage"
+      GCC   "--coverage -fno-inline -fno-inline-small-functions -fno-default-inline"
 )
 
 find_cppkg(benchmark 1.5.2  MODULE benchmark::benchmark TYPE lib OPTIONAL OFF)

flake.nix

@@ -0,0 +1,34 @@
+# in flake.nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let
+          overlays = [];
+          pkgs = import nixpkgs {
+            inherit system overlays;
+          };
+        in
+        with pkgs;
+        {
+          devShells.default = mkShell {
+            nativeBuildInputs = [
+              #llvmPackages_17.stdenv
+              gcc13Stdenv
+              ccache
+              cmake
+              ninja
+              lcov
+            ];
+            shellHook = ''
+              export CC=gcc
+              export CXX=g++
+            '';
+          };
+        }
+    );
+}