build: Use cp with -R instead of -r

The former is more portable and has not been marked as deprecated by
POSIX.

debian/changelog

@@ -24,6 +24,8 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
       only if available.
     - Check for the required minimal perl version.
     - Use builddir instead of CURDIR in man Makefile.am.
+    - Use cp with -R instead of -r (the former is more portable and not
+      marked as deprecated by POSIX).
 
   [ Updated manpages translations ]
   * German (Helge Kreutzmann).

man/Makefile.am

@@ -147,5 +147,5 @@ dist-hook: man.stamp
 	  cp $(srcdir)/po/$$lang.po $(distdir)/po; \
 	  cp $(srcdir)/po/$$lang.add $(distdir)/po; \
 	  $(MKDIR_P) $(distdir)/$$lang; \
-	  cp -r $(srcdir)/$$lang $(distdir)/; \
+	  cp -R $(srcdir)/$$lang $(distdir)/; \
 	done