office-ispell: Use a POSIX macro to enable including <termios.h>

Compatible with musl in particular. <unistd.h> is already included in
"config.h".

FreeBSD/NetBSD/OpenBSD/Apple (XNU) provides the macro too but are left
untouched as I don't have the means to test it.

Duplicate of an abandoned patch from 2015: https://reviews.llvm.org/D13938

MultiSource/Benchmarks/MiBench/office-ispell/term.c

@@ -76,7 +76,7 @@ static char Rcs_Id[] =
 #include "msgs.h"
 #include <signal.h>
 
-#if defined(__GLIBC__) || defined(__FreeBSD__) || defined(__NetBSD__) || \
+#if defined(_POSIX_VERSION) || defined(__FreeBSD__) || defined(__NetBSD__) || \
     defined(__OpenBSD__) || defined(__APPLE__) || \
     (defined(__sun__) && defined(__svr4__))
 #include <termios.h>