fixing abs() for gcc
fixing abs() for gcc

iamciera/IBAMR,yaojieuh/IBAMR,iamciera/IBAMR,huahbo/IBAMR,iamciera/IBAMR,yaojieuh/IBAMR,huahbo/IBAMR,huahbo/IBAMR,yaojieuh/IBAMR,iamciera/IBAMR,yaojieuh/IBAMR,huahbo/IBAMR,yaojieuh/IBAMR,huahbo/IBAMR,iamciera/IBAMR,yaojieuh/IBAMR,iamciera/IBAMR,huahbo/IBAMR,iamciera/IBAMR,yaojieuh/IBAMR,huahbo/IBAMR