Add missing PO variables

--- po/Makevars.orig	2017-04-21 20:45:54 UTC
+++ po/Makevars
@@ -53,3 +53,15 @@ MSGID_BUGS_ADDRESS = lftp-bugs@lftp.yar.ru
 # This is the list of locale categories, beyond LC_MESSAGES, for which the
 # message catalogs shall be used.  It is usually empty.
 EXTRA_LOCALE_CATEGORIES = 
+
+# This tells whether or not to regenerate a PO file when $(DOMAIN).pot
+# has changed.  Possible values are "yes" and "no".  Set this to no if
+# the POT file is checked in the repository and the version control
+# program ignores timestamps.
+PO_DEPENDS_ON_POT = yes
+
+# This tells whether or not to forcibly update $(DOMAIN).pot and
+# regenerate PO files on "make dist".  Possible values are "yes" and
+# "no".  Set this to no if the POT file and PO files are maintained
+# externally.
+DIST_DEPENDS_ON_UPDATE_PO = yes