maint: refactor SMACK interface to a separate module

Consolidate all smack routines and checks in a module.
We replace and wrap the most commonly used smack routines,
which allows removing ifdefs throughout the code.

* gl/lib/smack.h: A new header containing the implementation
of the wrapped and replacement routines.  Note the is_smack_enabled()
routine should be optimized out at compile time when compiled
on a system without libsmack.
* gl/modules/smack: Describe the new module and move the
configure time code here from ...
* m4/jm-macros.m4: ... here.
* bootstrap.conf: Reference the new module.
* src/id.c: Use the routines without ifdefs where possible.
* src/ls.c: Likewise.
* src/mkdir.c: Likewise.
* src/mkfifo.c: Likewise.
* src/mknod.c: Likewise.

gl/modules/smack

@@ -0,0 +1,42 @@
+Description:
+Include and determine the availability of smack routines
+
+Files:
+lib/smack.h
+
+Depends-on:
+
+configure.ac:
+# Check whether libsmack is available
+LIB_SMACK=
+AC_ARG_ENABLE([libsmack],
+  AC_HELP_STRING([--disable-libsmack], [disable libsmack support]))
+if test "X$enable_libsmack" != "Xno"; then
+  AC_CHECK_LIB([smack], [smack_new_label_from_self],
+    [AC_CHECK_LIB([smack], [smack_new_label_from_path],
+      [AC_CHECK_HEADER([sys/smack.h],
+        [LIB_SMACK=-lsmack
+         AC_DEFINE([HAVE_SMACK], [1], [libsmack usability])]
+      )])])
+  if test "X$LIB_SMACK" = "X"; then
+    if test "X$enable_libsmack" = "Xyes"; then
+      AC_MSG_ERROR([libsmack library was not found or not usable])
+    fi
+  fi
+else
+  AC_MSG_WARN([libsmack support disabled by user])
+fi
+AC_SUBST([LIB_SMACK])
+
+
+Makefile.am:
+lib_SOURCES += smack.h
+
+Include:
+"smack.h"
+
+License:
+LGPL
+
+Maintainer:
+Pádraig Brady