should be it

external/duckdb/extension/demo_capi/CMakeLists.txt

@@ -0,0 +1,27 @@
+cmake_minimum_required(VERSION 2.8.12...3.29)
+
+project(DemoCapiExtension)
+
+include_directories(include)
+
+# Configure extension: the minimum CAPI version where the extension still
+# compiles should be chosen here
+set(EXTENSION_NAME demo_capi)
+
+set(EXTENSION_FILES add_numbers.cpp capi_demo.cpp)
+
+option(
+  USE_UNSTABLE_C_API
+  "Use the unstable C Extension API (extension will be tied to exact DuckDB version)"
+  TRUE)
+
+if(USE_UNSTABLE_C_API)
+  build_loadable_extension_capi_unstable(${EXTENSION_NAME} ${EXTENSION_FILES})
+else()
+  # Minimum supported DuckDB version
+  set(CAPI_MAJOR 1)
+  set(CAPI_MINOR 2)
+  set(CAPI_PATCH 0)
+  build_loadable_extension_capi(${EXTENSION_NAME} ${CAPI_MAJOR} ${CAPI_MINOR}
+                                ${CAPI_PATCH} ${EXTENSION_FILES})
+endif()