bugfix GNU_SOURCE

There was a bug in GNU_SOURCE, which would prevent the following patch doing the job correctly:
https://github.com/fgeorgatos/easybuild-easyconfigs/commit/d281a18d69141894592acaf73e0a7ba1e8e9ff9e
(and in fact is required for all builds of GNU tools). I'd consider this is as hotfix and required for v1.10;

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>