Tell ceedling to use the uuid library when linking

The uuid library is required by some unit tests for blktap.

Signed-off-by: Flavien Quesnel <flavien.quesnel@citrix.com>
Define _GNU_SOURCE for blktap

_GNU_SOURCE is defined/required by several source files and make files
in blktap.

Signed-off-by: Flavien Quesnel <flavien.quesnel@citrix.com>