copysignf: Don't override undeclared system function on IRIX 6.5.

copysignf: Don't override undeclared system function on IRIX 6.5.

* lib/math.in.h (copysignf): Test HAVE_DECL_COPYSIGNF, not
HAVE_COPYSIGNF.
* m4/copysignf.m4 (gl_FUNC_COPYSIGNF): Don't assume copysignf() is
declared when it exists. Set HAVE_DECL_COPYSIGNF.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize HAVE_DECL_COPYSIGNF,
not HAVE_COPYSIGNF.
* modules/math (Makefile.am): Substitute HAVE_DECL_COPYSIGNF, not
HAVE_COPYSIGNF.
* doc/posix-functions/copysignf.texi: Mention missing declaration
problem.