added tests showing difference with Annex G, see comment 7 in
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57994
atan, atanh: Fix discrepancy with C standard for (+-0, +-1).

atan.c: Fix special case.
atan.dat: Fix tests.
Update NEWS.
added examples where mpc is slow (from discussion with Joseph S. Myers)
added pointer
added another link