jq: compile as _GNU_SOURCE to fix segfault when compiled with gcc 6

jq: compile as _GNU_SOURCE to fix segfault when compiled with gcc 6

When compiling host-jq with gcc 6+, running it gives an immediate segfault.
Reported upstream: https://github.com/stedolan/jq/issues/1598

The issue can be solved by compiling with _GNU_SOURCE as extra preprocessor
define. Once the issue is solved upstream, this change can be reverted.

As the issue will normally be the same for target, apply the same fix there.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>