maint: don't use gnulib's pathmax module; define PATH_MAX if needed

* bootstrap.conf (gnulib_modules): Remove pathmax.
* src/system.h: Don't include "pathmax.h".
(PATH_MAX) [!PATH_MAX]: Define to 8192.  Defining it to a constant
is preferable to using a definition from pathmax.h that might expand
to pathconf ("/", _PC_PATH_MAX). Prompted by discussion leading to:
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/27183/focus=27269

bootstrap.conf

@@ -158,7 +158,6 @@ gnulib_modules="
   nproc
   obstack
   parse-datetime
-  pathmax
   perl
   physmem
   posix-shell

src/system.h

@@ -40,11 +40,10 @@ you must include <sys/types.h> before including this file
 
 #include <unistd.h>
 
-/* limits.h must come before pathmax.h because limits.h on some systems
-   undefs PATH_MAX, whereas pathmax.h sets PATH_MAX.  */
 #include <limits.h>
-
-#include "pathmax.h"
+#ifndef PATH_MAX
+# define PATH_MAX 8192
+#endif
 
 #include "configmake.h"