Add main.c, modify ci.yml

.gitea/workflows/ci.yml

@@ -4,5 +4,7 @@ jobs:
     build:
         runs-on: ubuntu-latest
         steps:
-          - name: Printing hello
-            run: echo "Hello, "
+          - uses: actions/checkout@v4
+          - name: build using gcc
+            run: gcc -Wall -Werror main.c
+

.gitea/workflows/main.c

@@ -0,0 +1,5 @@
+int main()
+{
+	printf("Halo, Welt!");
+	return 0;
+}