build: use HAVE_LIBGEN_H consistently

Also, we are _GNU_SOURCE, so simplify the conditions accordingly.

The next patch will remove _GNU_SOURCE everywhere else.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
5 files changed