cbrtf: Work around bug in IRIX 6.5 system function.

cbrtf: Work around bug in IRIX 6.5 system function.

* lib/math.in.h (cbrtf): Override if REPLACE_CBRTF is 1.
* m4/cbrtf.m4 (gl_FUNC_CBRTF_WORKS): New macro.
(gl_FUNC_CBRTF): Invoke it. Set REPLACE_CBRTF to 1 if cbrtf() does not
work.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_CBRTF.
* modules/math (Makefile.am): Substitute REPLACE_CBRTF.
* modules/cbrtf (configure.ac): Consider REPLACE_CBRTF.
(Depends-on): Update conditions.
* doc/posix-functions/cbrtf.texi: Mention the IRIX 6.5 problem.