Workaround for PATH_MAX usage on the Hurd.

Taken from https://salsa.debian.org/debian/screen/-/blob/master/debian/patches/61-default-PATH_MAX-if-undefined-for-hurd.patch

Description: Set PATH_MAX to 4096 if undefined
 Fixes FTBFS since 4.4.0 on GNU/Hurd. Updated to add one more occurrence for 4.5.0.
Author: Axel Beckert <abe@debian.org>
Bug: https://savannah.gnu.org/bugs/?50089
Last-Updated: 2017-01-18

--- a/tty.sh
+++ b/tty.sh
@@ -1478,6 +1478,13 @@
   return 0;
 }
 
+/*
+ * Define PATH_MAX to 4096 if it's not defined, like on GNU/Hurd
+ */
+
+#ifndef PATH_MAX
+#define PATH_MAX 4096
+#endif
 
 int CheckTtyname (char *tty)
 {
--- a/screen.h
+++ b/screen.h
@@ -109,6 +109,13 @@
 # define DEFAULT_BUFFERFILE	"/tmp/screen-exchange"
 #endif
 
+/*
+ * Define PATH_MAX to 4096 if it's not defined, like on GNU/Hurd
+ */
+
+#ifndef PATH_MAX
+#define PATH_MAX 4096
+#endif
 
 #if defined(hpux) && !(defined(VSUSP) && defined(VDSUSP) && defined(VWERASE) && defined(VLNEXT))
 # define HPUX_LTCHARS_HACK