maint: ensure PATH_MAX is set correctly

This reverts part of commit v8.12-103-g54cbe6e.

* src/system.h: Include gnulib's pathmax.h to honor
system specific limits, and then we set PATH_MAX only if needed.
Note pathmax.h no longer uses pathconf ("/", _PC_PATH_MAX).
Note I didn't reinstate the comments about limits.h inclusion
order, because pathmax.h includes limits.h anyway.

bootstrap.conf

@@ -171,6 +171,7 @@ gnulib_modules="
   nproc
   obstack
   parse-datetime
+  pathmax
   perl
   physmem
   pipe

src/system.h

@@ -41,6 +41,8 @@ you must include <sys/types.h> before including this file
 #include <unistd.h>
 
 #include <limits.h>
+
+#include "pathmax.h"
 #ifndef PATH_MAX
 # define PATH_MAX 8192
 #endif