* lib/math.in.h (cbrtf): New declaration.
* lib/cbrtf.c: New file.
* m4/cbrtf.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether cbrtf is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_CBRTF, HAVE_CBRTF,
HAVE_DECL_CBRTF.
* modules/math (Makefile.am): Substitute GNULIB_CBRTF, HAVE_CBRTF,
HAVE_DECL_CBRTF.
* modules/cbrtf: New file.
* tests/test-math-c++.cc: Check the declaration of cbrtf.
* doc/posix-functions/cbrtf.texi: Mention the new module.