seq: work around floating point inaccuracies on more systems

seq: work around floating point inaccuracies on more systems

* src/seq.c: Include <math.h> for fabs.
Include <float.h> for DBL_EPSILON.
(abs_rel_diff): New function.
(print_numbers): Use abs_rel_diff rather than a strict equality test.
Without this change, Solaris 8 and Irix 6.2 would fail the float-6
test.  Reported by Peter Fales in
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/13183
* src/c99-to-c89.diff: Adjust seq.c offsets.