samba: add option for built-in debug messages

This option allows users to specify the maximum level for compiled-in
debug messages with -1 being none.
In a typical scenario of nmbd+smbd+smbpasswd this can save a full
megabyte of target space.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>