build: Tell xgettext that the source is UTF-8.

* po/Makevars (XGETTEXT_OPTIONS): Add `--from-code=UTF-8'.

po/Makevars

@@ -7,7 +7,7 @@ top_builddir = ..
 
 # These options get passed to xgettext.
 XGETTEXT_OPTIONS =					\
-  --language=Scheme					\
+  --language=Scheme --from-code=UTF-8			\
   --keyword=_ --keyword=N_				\
   --keyword=description --keyword=long-description