init contrib_GMT

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>
Merge branch 'develop' of https://github.com/hpcugent/easybuild-easyconfigs into contrib_GMT
tell about Ghostscript dependency

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>
Merge pull request #560 from fgeorgatos/contrib_GMT

init contrib_GMT