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