preserve underlay file times

I'd like to preserve the mtime and ctime of files in the underlay if
possible, to avoid ikiwiki thinking they've changed and refreshing them
each time it's upgraded.

But, fall back to the old install command for systems that lack cp -a.

Makefile.PL

@@ -54,6 +54,7 @@ extra_install:
 	for dir in `cd underlays && find . -follow -type d ! -regex '.*\.svn.*'`; do \
 		install -d $(DESTDIR)$(PREFIX)/share/ikiwiki/$$dir; \
 		for file in `find underlays/$$dir -follow -maxdepth 1 -type f`; do \
+			cp -aL $$file $(DESTDIR)$(PREFIX)/share/ikiwiki/$$dir || \
 			install -m 644 $$file $(DESTDIR)$(PREFIX)/share/ikiwiki/$$dir; \
 		done; \
 	done