remainder: Support for MSVC.

remainder: Support for MSVC.

* lib/math.in.h (remainder): New declaration.
* lib/remainder.c: New file.
* m4/remainder.m4: New file.
* modules/remainder (Files): Add lib/remainder.c, m4/remainder.m4.
(Depends-on): Add math, round, fma.
(configure.ac): Use results of gl_FUNC_REMAINDER.
* m4/math_h.m4 (gl_MATH_H): Test whether remainder is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_REMAINDER, HAVE_REMAINDER,
HAVE_DECL_REMAINDER.
* modules/math (Makefile.am): Substitute GNULIB_REMAINDER,
HAVE_REMAINDER, HAVE_DECL_REMAINDER.
* tests/test-math-c++.cc: Check the declaration of remainder.
* doc/posix-functions/remainder.texi: Mention that the MSVC and IRIX 5
problems are fixed.