PATH_MAX is defined in limits.h.

--- src/pstree.c.orig	2014-11-10 15:26:59.195715584 +0100
+++ src/pstree.c	2014-11-10 15:27:20.666926426 +0100
@@ -38,6 +38,7 @@
 #include <termios.h>
 #include <langinfo.h>
 #include <assert.h>
+#include <limits.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>