adjust configopts as per #204

Signed-off-by: Fotis Georgatos <fotis.georgatos@uni.lu>
Merge branch 'develop' of github.com:hpcugent/easybuild-easyconfigs into 204_fix_OpenMPI_configure_mpirun_prefix
Merge pull request #205 from fgeorgatos/204_fix_OpenMPI_configure_mpirun_prefix

adjust OpenMPI configopts as per #204