New module 'copysignl'.

New module 'copysignl'.

* lib/math.in.h (copysignl): New declaration.
* lib/copysignl.c: New file.
* m4/copysignl.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether copysignl is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_COPYSIGNL, HAVE_COPYSIGNL.
* modules/math (Makefile.am): Substitute GNULIB_COPYSIGNL,
HAVE_COPYSIGNL.
* modules/copysignl: New file.
* tests/test-math-c++.cc: Check the declaration of copysignl.
* modules/math-c++-tests (Makefile.am): Link test-math-c++ against
$(COPYSIGNL_LIBM). Needed on IRIX 6.5 with cc.
* doc/posix-functions/copysignl.texi: Mention the new module.