cbrt: Provide replacement on MSVC and Minix.

cbrt: Provide replacement on MSVC and Minix.

* lib/math.in.h (cbrt): New declaration.
* lib/cbrt.c: New file.
* m4/cbrt.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether cbrt is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_CBRT, HAVE_CBRT.
* modules/math (Makefile.am): Substitute GNULIB_CBRT, HAVE_CBRT.
* modules/cbrt (Files): Add lib/cbrt.c, m4/cbrt.m4.
(Depends-on): Add dependencies.
(configure.ac): Arrange to compile replacement if HAVE_CBRT is 0.
* tests/test-math-c++.cc: Check the declaration of cbrt.
* doc/posix-functions/cbrt.texi: Mention that the module provides a
replacement.