[ccache] preprocessing switches

[Shentino]

Does ccache take macro switches into account when it preprocesses?

For example, -DRANDOM_MACRO 's side effect is purely to introduce said macro
during preprocessing but beyond that has no effect on actual compilation.

Yet I have a source file that doesn't define such a macro, and it doesn't
get cached.  However, the compiler generates the same object file with or
without it.