r15206: Heimdal is always compiled with _GNU_SOURCE enabled (in its configure.in...

r15206: Heimdal is always compiled with _GNU_SOURCE enabled (in its configure.in it is defined unconditionally).
However, the lex-generated files don't fetch config.h's definition of _GNU_SOURCE before including
features.h because of preamble added by lex which already includes some system headers which in turn
include features.h. Therefore, we need to specify it explicitly on comand line.

This change adds some 'noise' (_GNU_SOURCE already defined ...) but it is neccessary to get samba4 to
compile successfuly on Maemo.
(This used to be commit cd7307c099ace123d840459da39a49ccf3872267)