tests/eina: make inclusion for define explicit and more compatible

Summary:
signal.h is not included with all build options somehow, making this
a bit unreliable. also check for define existence for random platforms
which don't define this symbol

Reviewers: devilhorns, ManMower

Reviewed By: ManMower

Subscribers: ManMower, cedric, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D6535