log1p-ieee: Work around test failure on AIX 7.1 and HP-UX 11.

log1p-ieee: Work around test failure on AIX 7.1 and HP-UX 11.

* m4/log1p-ieee.m4: New file.
* m4/log1p.m4 (gl_FUNC_LOG1P): If gl_FUNC_LOG1P_IEEE is present, test
whether log1p works with a minus zero argument. Replace it if not.
* lib/math.in.h (log1p): Override if REPLACE_LOG1P is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_LOG1P.
* modules/math (Makefile.am): Substitute REPLACE_LOG1P.
* modules/log1p (configure.ac): Consider REPLACE_LOG1P.
(Depends-on): Update conditions.
* modules/log1p-ieee (Files): Add m4/log1p-ieee.m4, m4/minus-zero.m4,
m4/signbit.m4.
(configure.ac): Invoke gl_FUNC_LOG1P_IEEE.
* doc/posix-functions/log1p.texi: Mention the log1p-ieee module.