copysign: Provide replacement.

copysign: Provide replacement.

* lib/math.in.h (copysign): New declaration.
* lib/copysign.c: New file.
* m4/copysign.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether copysign is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_COPYSIGN, HAVE_COPYSIGN.
* modules/math (Makefile.am): Substitute GNULIB_COPYSIGN,
HAVE_COPYSIGN.
* modules/copysign (Description): Clarify.
(Files): Add lib/copysign.c, m4/copysign.m4.
(Depends-on): Add math, signbit.
(configure.ac): Invoke gl_FUNC_COPYSIGN, AC_LIBOBJ,
gl_MATH_MODULE_INDICATOR.
* tests/test-math-c++.cc: Check the declaration of copysign.
* doc/posix-functions/copysign.texi: Mention the effects of the module
on Minix and MSVC.