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