New module 'rintl'.

New module 'rintl'.

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