Unix Makefiles: Make fast local and strip install truly fast

When using "Unix Makefiles" generator, install/local/fast and install/strip/fast targets are not at all fast. They depend on install/local and install/strip targets respectively instead of preinstall/fast and don't contain any commands instead of repeating commands in install/local and install/strip. The issue was introduced by commits eef327b9 (local) and df8f5500 (strip).