Avoid 'cp -u' as it's not portable.

In this case we can unconditionally copy files.

Reported-by: Michał Górny
Bug: https://bugs.gentoo.org/655444
--- a/doc/Makefile.in
+++ b/doc/Makefile.in
@@ -28,3 +28,3 @@ RM_RF		= rm -rf
 CP_F		= cp -f
-CP_UF		= cp -uf
+CP_UF		= cp -f