support bazel complie this project and format code.

basic_content/extern/extern_c/BUILD

@@ -0,0 +1,14 @@
+# please run `bazel run basic_content/extern/extern_c:main`
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
+
+cc_library(
+    name = "add",
+    srcs = ["add.cpp"],
+    hdrs = ["add.h"],
+)
+
+cc_binary(
+    name = "main",
+    srcs = ["add.c"],
+    deps = [":add"],
+)

basic_content/extern/extern_c/add.cpp

@@ -1,5 +1,3 @@
 #include "add.h"
 
-int add(int x,int y) {
-    return x+y;
-}
+int add(int x, int y) { return x + y; }

basic_content/extern/extern_c/add.h

@@ -1,6 +1,6 @@
 #ifndef ADD_H
 #define ADD_H
 extern "C" {
-    int add(int x,int y);
+int add(int x, int y);
 }
 #endif