Don't rmdir the destination directory if it's a symlink during "make install-data-hook"

This fixes a build error on linux during subsequent "make install" runs after the first "make install" succeeds

Makefile.am

@@ -71,7 +71,7 @@ install-data-hook:
 	if test -d "$(DESTDIR)@includedir@/json" ; then \
 		(cd "$(DESTDIR)@includedir@/json" && \
 		rm -f $(libjson_cinclude_HEADERS)) ; \
-		rmdir "$(DESTDIR)@includedir@/json" ; \
+		test -L "$(DESTDIR)@includedir@/json" || rmdir "$(DESTDIR)@includedir@/json" ; \
 	fi
 	test \! -e "$(DESTDIR)@includedir@/json" || rm "$(DESTDIR)@includedir@/json"
 	$(LN_S) json-c "$(DESTDIR)@includedir@/json"