Here's the output of configure.
Just as Mark said, it looks this is due to a libm bug.

"""
$ grep tanh config.log
configure:22062: checking whether tanh preserves the sign of zero
|        and tanh(-0.) == -0. */
|       atan2(tanh(-0.), -1.) == atan2(-0., -1.)) exit(0);
configure:22261: checking for atanh
ac_cv_func_atanh=yes
ac_cv_tanh_preserves_zero_sign=no
"""