i18n: Add N_n() no-op macro to allow xgettext to extract plural forms

This is intended to allow xgettext to extract plural forms from the code
without actually altering the input. Basically, this is to _n() as N_()
is to _(), and doesn't actually call any gettext functions.

cmake/FindTranslationTools.cmake

@@ -26,6 +26,7 @@ set(GETTEXT_XGETTEXT_OPTIONS
 	--keyword=vgettext
 	--keyword=VGETTEXT
 	--keyword=_n:1,2
+	--keyword=N_n:1,2
 	--keyword=sngettext:1,2
 	--keyword=vngettext:1,2
 	--keyword=VNGETTEXT:1,2

po/SConscript

@@ -58,7 +58,7 @@ if "pot-update" in COMMAND_LINE_TARGETS:
                 """xgettext --force-po --default-domain=%s --directory=. --add-comments=TRANSLATORS: \
                 --from-code=UTF-8 --sort-by-file \
                 --keyword=_ --keyword=N_ --keyword=sgettext --keyword=vgettext --keyword=VGETTEXT \
-                --keyword=_n:1,2 --keyword=sngettext:1,2 --keyword=vngettext:1,2 --keyword=VNGETTEXT:1,2 \
+                --keyword=_n:1,2 --keyword=N_n:1,2 --keyword=sngettext:1,2 --keyword=vngettext:1,2 --keyword=VNGETTEXT:1,2 \
                 --files-from=%s --copyright-holder='Wesnoth development team' --msgid-bugs-address=https://bugs.wesnoth.org/ \
                 --output=$TARGET \
                 ; sed -i s/charset=CHARSET/charset=UTF-8/ $TARGET \

src/gettext.hpp

@@ -99,3 +99,4 @@ namespace translation
 
 #define gettext_noop(String) String
 #define N_(String) gettext_noop (String)
+#define N_n(String1, String2) String1, String2