Warn about deprepcated build options

[starbelly]

We will continue to allow include_files and exclude_regexps for some time, yet we need to steer people further from these options and towards include_paths and exclude_paths (including myself 😛) loudly.

[starbelly]

Minor language tweaks, but I assume you'll know better than me 😄

I made one more tweak, which was to remove the 's. It's confusing, but an object can not have a possessive form.