#157 isatty returns wrong value when stdout is redirected to /dev/null

[Daiki Ueno]

With msys2-runtime 2.1.0.16305.2579661-2 on 64-bit Windows 8.1, isatty returns 64 when stdout is redirected to /dev/null:

$ cat test-isatty.c
#include <stdio.h>
#include <unistd.h>
int main (void) { fprintf (stderr, "%d\n", isatty (STDOUT_FILENO)); return 0; }

$ gcc -o test-isatty test-isatty.c
$ ./test-isatty >a
0
$ ./test-isatty >/dev/null
64

This leads to a crash of an xgettext call in gtk+'s configure.ac:
https://github.com/Alexpux/MINGW-packages/blob/master/mingw-w64-gtk3/do-not-check-xgettext.patch
which is now disabled.