build: allow unconditionally enabling Modem.Command() method

If ModemManager is being used in a custom system, make it easy to
unconditionally enable the Modem.Command() method, in case the modem
needs to be configured in some way that is not supported via standard
modem interfaces.

configure.ac

@@ -313,6 +313,23 @@ fi
 
 AM_CONDITIONAL(WITH_POLKIT, [test "x$with_polkit" != "xno"])
 
+dnl-----------------------------------------------------------------------------
+dnl AT command via DBus support (disabled by default unless running in --debug)
+dnl
+dnl It is suggested that this option is only enabled in custom built systems and
+dnl only if truly required.
+dnl
+
+AC_ARG_WITH(at_command_via_dbus,
+            AS_HELP_STRING([--with-at-command-via-dbus],
+                           [Build with Modem.Command() interface enabled always]),
+            [],
+            [with_at_command_via_dbus=no])
+
+if test "x$with_at_command_via_dbus" = "xyes"; then
+    AC_DEFINE(WITH_AT_COMMAND_VIA_DBUS, 1, [Define if you want to enable AT commands via DBus])
+fi
+
 dnl-----------------------------------------------------------------------------
 dnl MBIM support (enabled by default)
 dnl
@@ -444,6 +461,7 @@ echo "
       qmi:                     ${with_qmi}
       systemd suspend/resume:  ${with_systemd_suspend_resume}
       systemd journal:         ${with_systemd_journal}
+      at command via dbus:     ${with_at_command_via_dbus}
 
     Miscellaneous:
       gobject introspection:   ${found_introspection}

src/mm-iface-modem.c

@@ -753,6 +753,7 @@ handle_command_auth_ready (MMBaseModem *self,
         return;
     }
 
+#if ! defined WITH_AT_COMMAND_VIA_DBUS
     /* If we are not in Debug mode, report an error */
     if (!mm_context_get_debug ()) {
         g_dbus_method_invocation_return_error (ctx->invocation,
@@ -763,6 +764,7 @@ handle_command_auth_ready (MMBaseModem *self,
         handle_command_context_free (ctx);
         return;
     }
+#endif
 
     /* If command is not implemented, report an error */
     if (!MM_IFACE_MODEM_GET_INTERFACE (self)->command ||