hypot-ieee: Work around test failure on OSF/1 and native Windows.

hypot-ieee: Work around test failure on OSF/1 and native Windows.

* lib/math.in.h (hypot): New declaration.
* lib/hypot.c: New file.
* m4/hypot-ieee.m4: New file.
* m4/hypot.m4 (gl_FUNC_HYPOT): If gl_FUNC_HYPOT_IEEE is present, test
whether hypot works with mixed NaN and Infinity arguments. Replace it
if not.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize GNULIB_HYPOT,
REPLACE_HYPOT.
* modules/math (Makefile.am): Substitute GNULIB_HYPOT, REPLACE_HYPOT.
* modules/hypot (Files): Add lib/hypot.c.
(Depends-on): Add dependencies.
(configure.ac): Arrange to compile replacement if REPLACE_HYPOT is 1.
* modules/hypot-ieee (Files): Add m4/hypot-ieee.m4.
(configure.ac): Invoke gl_FUNC_HYPOT_IEEE.
* tests/test-math-c++.cc: Check the declaration of hypot.
* doc/posix-functions/hypot.texi: Mention the hypot-ieee module.