/in/foo.cc: In function 'int CountLower(char*)':
/in/foo.cc:8:23: error: 'strlen' was not declared in this scope
     int len = strlen(s);
                       ^
/in/foo.cc: In function 'int CountUpper(char*)':
/in/foo.cc:19:23: error: 'strlen' was not declared in this scope
     int len = strlen(s);
                       ^
/in/foo.cc: In function 'void Concat(char*, char*, char*)':
/in/foo.cc:37:25: error: 'strlen' was not declared in this scope
     int len1 = strlen(s1);
                         ^
/in/foo.cc:45:22: error: 'strcat' was not declared in this scope
     strcat(s3, s2 + 1);
                      ^
/in/foo.cc:38:9: warning: unused variable 'len2' [-Wunused-variable]
     int len2 = strlen(s2);
         ^~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:52:5: error: 'cin' was not declared in this scope
     cin.getline(s1, N);
     ^~~
/in/foo.cc:56:2: error: 'cout' was not declared in this scope
  cout <<" " << s3 << endl;
  ^~~~
/in/foo.cc:56:22: error: 'endl' was not declared in this scope
  cout <<" " << s3 << endl;
                      ^~~~