build: separate compiler and linker flags

-Wall is a compiler flag, and thus should be in CFLAGS,
not CPPFLAGS. Similarly, -l belongs into LDADD/LIBADD because of
ordering constraints.

Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
attr: Allow attribute type 0

{netlink,packet,unix}_diag use attribute type 0 for valid
attributes. The value was reserved and usage was prohibited
by the protocol but we can't undo the breakge.

Make libnl accept attribute type 0 to allow parsing these
attributes.

Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>