Add gpfs libraries if needed

Last commit introduced gpfs_fcntl operations, depending if gpfs.h and
gpfs_fcntl.h were found.  These routines need the gpfs library.  This
configure check will bring in libgpfs -- you will still need to set
LDFLAGS and CPPFLAGS if gpfs is installed in some non-default locaiton
(as it is on Blue Gene).