small patch for Mingw w64 32bit build

[Tatsuro M. <tma...@ya...>]

I have submitted a small patch for Mingw w64 32bit build.

Please see
https://sourceforge.net/p/gnuplot/bugs/1791/

I also write it here

Tatsuro

--- a/src/internal.c    2016-04-27 01:46:22.000000000 +0900 +++ b/src/internal.c    2016-05-08 16:09:44.181377500 +0900 @@ -47,7 +47,7 @@ #include <math.h> -#ifndef _WIN64 +#if !defined (_WIN64) && !defined (__MINGW64_VERSION_MAJOR) /* * FIXME: This is almost certainly out of date on linux, since the matherr * mechanism has been replaced by math_error() and supposedly is only