Fix use_ccache and allow_parallel_build defaults
cerbero: Enable ccache by default on all platforms
In practice this will only be used when building on Linux, or when
building on macOS with ccache installed via Homebrew.
cerbero: Clean up allow_parallel_build defaults
It's enabled by default everywhere except when running on Windows.
CC: @ystreet you mentioned there were some issues enabling ccache on macOS, should we not do that?