no hardcoding of gpp command

Signed-off-by: Mattias Andrée <maandree@operamail.com>
whitespace

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add sentinel singly linked list

Signed-off-by: Mattias Andrée <maandree@operamail.com>
add sentinel doubly linked list

Signed-off-by: Mattias Andrée <maandree@operamail.com>