Question about libpq++

I've installed pgsql 6.4.2
It includes libpq++
I've tried to play with it...
Simply
#include <stdlib.h>
#include <libpq++.h>
int main(void)
{
      PgEnv a;
      a.SetEnv(NULL,NULL,NULL,NULL,NULL);
      return 0;
}

during compilation with "g++ -o pgsetenv pgsetenv.cpp
-I/usr/local/pgsql/include" I've got an error message:
/var/tmp/ccxg22111.o: Undefined symbol `___5PgEnv' referenced from text segment
....

What to do?

thanks,
       Q