docs.mk: use posix arguments for cp

`cp -a` is a GNU extension. Use an alternate combinaison of POSIX options
(`-PRp`) that do nearly the same.

The difference is `-a` will preserve context, links and xattr attributes,
whereas `-p` not. But as we use it only for copy a file, there is no
difference in the current context.

Author: semarie

mk/docs.mk

@@ -129,21 +129,21 @@ doc/:
 HTML_DEPS += doc/rust.css
 doc/rust.css: $(D)/rust.css | doc/
 	@$(call E, cp: $@)
-	$(Q)cp -a $< $@ 2> /dev/null
+	$(Q)cp -PRp $< $@ 2> /dev/null
 
 HTML_DEPS += doc/favicon.inc
 doc/favicon.inc: $(D)/favicon.inc | doc/
 	@$(call E, cp: $@)
-	$(Q)cp -a $< $@ 2> /dev/null
+	$(Q)cp -PRp $< $@ 2> /dev/null
 
 doc/full-toc.inc: $(D)/full-toc.inc | doc/
 	@$(call E, cp: $@)
-	$(Q)cp -a $< $@ 2> /dev/null
+	$(Q)cp -PRp $< $@ 2> /dev/null
 
 HTML_DEPS += doc/footer.inc
 doc/footer.inc: $(D)/footer.inc | doc/
 	@$(call E, cp: $@)
-	$(Q)cp -a $< $@ 2> /dev/null
+	$(Q)cp -PRp $< $@ 2> /dev/null
 
 # The (english) documentation for each doc item.