LocalGenerator: Add a string overload for AppendFlags
ninja: Factor out target-level order-only dependencies

This reduces ninja file output even more for projects with lots of
libraries with entangled transitive dependencies. ParaView goes from the
previous 58M to about 45M.
ninja: Factor out custom command order-only depends

This makes WebKitGTK's CMake build.ninja file go from 165M to 11M and
configure/generate in 5 seconds.
LocalGenerator: Fix -Woverload-virtual warning
Fix 'using' statement