log1pl-ieee: Work around test failure on IRIX 6.5.

log1pl-ieee: Work around test failure on IRIX 6.5.

* m4/log1pl-ieee.m4: New file.
* m4/log1pl.m4 (gl_FUNC_LOG1PL): If gl_FUNC_LOG1PL_IEEE is present,
test whether log1pl works with a minus zero argument. Replace it if
not.
* lib/math.in.h (log1pl): Override if REPLACE_LOG1PL is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_LOG1PL.
* modules/math (Makefile.am): Substitute REPLACE_LOG1PL.
* modules/log1pl (configure.ac): Consider REPLACE_LOG1PL.
(Depends-on): Update conditions.
* modules/log1pl-ieee (Files): Add m4/log1p-ieee.m4, m4/minus-zero.m4,
m4/signbit.m4.
(configure.ac): Invoke gl_FUNC_LOG1PL_IEEE.
* doc/posix-functions/log1pl.texi: Mention the log1pl-ieee module.