rint: Support for MSVC.

rint: Support for MSVC.

* lib/math.in.h (rint): New declaration.
* lib/rint.c: New file.
* m4/rint.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether rint is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_RINT, HAVE_RINT.
* modules/math (Makefile.am): Substitute GNULIB_RINT, HAVE_RINT.
* modules/rint (Description): Fix.
(Files): Add lib/rint.c, m4/rint.m4.
(Depends-on): Add math.
(configure.ac): Invoke gl_FUNC_RINT, AC_LIBOBJ,
gl_MATH_MODULE_INDICATOR.
* tests/test-math-c++.cc: Check the declaration of rint.
* modules/math-c++-tests (Makefile.am): Link test-math-c++ against
$(RINT_LIBM). Needed on IRIX 6.5 with cc.
* doc/posix-functions/rint.texi: Mention the replacement provided by
the module.