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

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

* m4/cbrtl-ieee.m4: New file.
* m4/cbrtl.m4 (gl_FUNC_CBRTL): If gl_FUNC_CBRTL_IEEE is present,
test whether cbrtl works with a minus zero argument. Replace it if not.
* lib/math.in.h (cbrtl): Override if REPLACE_CBRTL is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_CBRTL.
* modules/math (Makefile.am): Substitute REPLACE_CBRTL.
* modules/cbrtl (configure.ac): Consider REPLACE_CBRTL.
(Depends-on): Update conditions.
* modules/cbrtl-ieee (Files): Add m4/cbrtl-ieee.m4, m4/minus-zero.m4,
m4/signbit.m4.
(configure.ac): Invoke gl_FUNC_CBRTL_IEEE.
* lib/cbrtl.c (cbrtl) [IRIX]: Avoid an unnecessary addition.
* doc/posix-functions/cbrtl.texi: Mention the cbrtl-ieee module.