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 -rfCP_F = cp -f
-CP_UF = cp -uf+CP_UF = cp -f