mu: compile pop support conditionally.

* mu/Makefile.am [MU_COND_SUPPORT_POP]: Compile pop.c conditionally.
* mu/mu.c (mutool_nosys): New function.
(mutool_action_tab): Compile pop support conditionally.

@@ -16,6 +16,13 @@
 ## along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 bin_PROGRAMS = mu
+
+if MU_COND_SUPPORT_POP
+ POP_C=pop.c
+endif
+
+EXTRA_DIST=pop.c
+
 mu_SOURCES = \
  acl.c\
  info.c\
@@ -23,7 +30,7 @@ mu_SOURCES = \
  mu.c\
  filter.c\
  flt2047.c\
- pop.c\
+ $(POP_C)\
  query.c\
  shell.c
 

@@ -87,9 +87,21 @@ struct mutool_action_tab
   mutool_action_t action;
 };
 
+static int
+mutool_nosys (int argc, char **argv)
+{
+  mu_error (_("%s is not available because required code is not compiled"),
+	    argv[0]);
+  return 1;
+}
+
 struct mutool_action_tab mutool_action_tab[] = {
   { "info", mutool_info },
+#ifdef ENABLE_POP
   { "pop", mutool_pop },
+#else
+  { "pop", mutool_nosys },
+#endif
   { "filter", mutool_filter },
   { "2047", mutool_flt2047 },
   { "query", mutool_query },