oonf_api requires undocumented define

[OlegHahm]

Building an application (e.g. https://github.com/Lotterleben/RIOT-AODVv2/tree/master/aodvv2_demo) with oonf_api fails with an error like

In file included from /home/oleg/git/RIOT/pkg/oonf_api/oonf_api/src-api/common/avl_comp.h:46:0,
                 from /home/oleg/git/RIOT/pkg/oonf_api/oonf_api/src-api/common/avl_comp.c:45:
/home/oleg/git/RIOT/pkg/oonf_api/oonf_api/src-api/common/netaddr.h: In function ‘netaddr_socket_get_addressfamily’
/home/oleg/git/RIOT/pkg/oonf_api/oonf_api/src-api/common/netaddr.h:348:16: error: ‘sockaddr6_t’ has no member named ‘__in6_a’
   return s->std.sin6_family;

if one does not define the macro RIOT (which is done by the given example in the application Makefile by
export CFLAGS += -DRIOT

I think this define is useless, because it's only used in a RIOT specific patch (https://github.com/RIOT-OS/RIOT/blob/master/pkg/oonf_api/0001-add-RIOT-support.patch#L87) that can live without this ifdeff since the package inside RIOT will always be built for RIOT.

If there's still a reason for this define, it should be either documented somewhere and get a better name.

[OlegHahm]

Thanks to @mfrey for detecting this.

[Lotterleben]

Seems reasonable to me. @benpicco, since you're the author of this patch, can you tell us if you had any other intentions we're overlooking here?

[benpicco]

I think the intention was to have this always defined by a core RIOT Makefile, but that never got merged. I wanted to compile the same source code for RIOT and for Linux, so I needed a way to identify the platform - maybe something like this has been added by now? Otherwise it can't hurt to add such define to Makefile.include, better name it __RIOT__ for consistency.

[OlegHahm]

I'm not sure if we need this. Is it really a use case to build a thirdparty library inside RIOT (i.e. in $(RIOTBASE)/pkg/) for Linux?

[benpicco]

Well it's possible that one day such patches could be merged upstream, then they mustn't break the build for non-RIOT targets.

[OlegHahm]

Ok, makes sense. Let's close this as soon as #2805 is merged.

[Lotterleben]

Will keep an eye on it.