build: Pass --previous to msgmerge with the new MSGMERGE_OPTIONS variable

This new options got introduced in gettext 0.18, and helps significantly
translators' work.

dselect/po/Makevars

@@ -10,6 +10,8 @@ top_builddir = ../..
 # These options get passed to xgettext.
 XGETTEXT_OPTIONS = --keyword=_ --keyword=N_
 
+MSGMERGE_OPTIONS = --previous
+
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding
 # package.  (Note that the msgstr strings, extracted from the package's

po/Makevars

@@ -10,6 +10,8 @@ top_builddir = ..
 # These options get passed to xgettext.
 XGETTEXT_OPTIONS = --keyword=_ --keyword=N_ --keyword=_g --from-code=UTF-8
 
+MSGMERGE_OPTIONS = --previous
+
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding
 # package.  (Note that the msgstr strings, extracted from the package's

scripts/po/Makevars

@@ -10,6 +10,8 @@ top_builddir = ../..
 # These options get passed to xgettext.
 XGETTEXT_OPTIONS = --from-code=iso-8859-1 --keyword=_g
 
+MSGMERGE_OPTIONS = --previous
+
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding
 # package.  (Note that the msgstr strings, extracted from the package's