ccache docs should provide non-environment variable method

See https://mesonbuild.com/Feature-autodetection.html#ccache, which currently just mentions the `CC` and `CXX` environment variables. They are also lower case for some reason, and `build.*` / `*_FOR_BUILD` could also be mentioned.

Thanks to @baryluk in https://github.com/mesonbuild/meson/issues/7284#issuecomment-641660087 for the tip-off.

Start with the linked Feature-autodetection.html#ccache section and inspect the surrounding Meson documentation for the supported non-environment-variable approach. Done means the ccache section explains that method, corrects the variable casing, and mentions build.* and *_FOR_BUILD where applicable.

Written by the indexing model from the issue text.