Add main.c and update CI to build with gcc

.gitea/workflows/ci.yml

@@ -2,7 +2,7 @@ name: First CI
 on: [push]
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
       - name: build using gcc