wafsamba/samba_autoconf: when setting undefined result, use empty tuple

wafsamba/samba_autoconf: when setting undefined result, use empty tuple

A difference between waf 1.x and 2.x is that we gained 0 as an undefined
variable in the cache file. This does not allow to differentiate unset
and set to 0 defines.

Force to use empty tuple () to signify unset defines.

Also, fix handling of extra cflags in case of 'strict=True': if
extra_cflags were not defined, we'd append None to the cflags list and
it confuses conf.check() later. 'None' is added to the command line of a
tool executed by the conf.check() which, depending on a tool, may be
treated as an error and cause wrong test result.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>