Fix detection of openpty with gcc 4.

Index: Makefile.PL
--- Makefile.PL.orig
+++ Makefile.PL
@@ -179,20 +179,8 @@ foreach my $f (sort keys %funcs) {
 #include <fcntl.h>
 
 /* Try platform-specific headers for openpty and friends */
-#if defined(__has_include)
-#  if __has_include(<pty.h>)
-#    include <pty.h>
-#  endif
-#  if __has_include(<libutil.h>)
-#    include <libutil.h>
-#  endif
-#  if __has_include(<termios.h>)
 #    include <termios.h>
-#  endif
-#  if __has_include(<util.h>)
 #    include <util.h>
-#  endif
-#endif
 
 int
 main ()
