$ pwd
/usr/ports/pobj/etl-0.04.19/ETL-0.04.19/test
$ cat m.cc
#include <iostream>

int
main()
{
    return 0;
}
$
$ c++ -I../ETL -c m.cc
In file included from m.cc:1:
In file included from /usr/include/c++/v1/iostream:38:
In file included from /usr/include/c++/v1/ios:216:
/usr/include/c++/v1/__locale:140:5: error: unknown type name 'once_flag'
    once_flag      __flag_;
    ^
1 error generated.
$ c++ -c m.cc
$ echo $?
0
$