Don't use the system-helper if disabled

I.e. if there is a system-installed helper, then we should not use
it if flatpak was built without the system helper.

common/flatpak-dir.c

@@ -303,6 +303,11 @@ flatpak_dir_use_system_helper (FlatpakDir *self)
 {
   FlatpakSystemHelper *system_helper;
 
+#ifndef USE_SYSTEM_HELPER
+  if (TRUE)
+    return FALSE;
+#endif
+
   if (self->no_system_helper || self->user || getuid () == 0)
     return FALSE;
 

configure.ac

@@ -145,6 +145,7 @@ AC_ARG_ENABLE([system-helper],
 if test "x$enable_system_helper" = "xyes"; then
   PKG_CHECK_MODULES(POLKIT, \
   		    polkit-gobject-1 >= $POLKIT_GOBJECT_REQUIRED)
+  AC_DEFINE([USE_SYSTEM_HELPER], [1], [Define if using system-helper])
 fi
 AM_CONDITIONAL(BUILD_SYSTEM_HELPER, test x$enable_system_helper = xyes)