Draft of an potential interface for `--check-cfg` in `Cargo.toml`

Cargo.check-cfg.toml

[package]
# ...

[check-cfg] # or [package.check-cfg] ?
well-known = true   # this only and exclusively control the names() part
                    # aka as the well-known names (or values, maybe) in rustc
                    # this default to true
names = ["no_global_oom_handling"] # this is the names(...) part, this
                                   # only check the name part of the cfg
                                   # and never the value of it (except if
                                   # defined below in values, or by cmd)

[[check-cfg.values]] # this is the values(...) part
name = "use_libc" # this is not equivalent to just putting the name in
                  # package.cfg.check.names as this defines an implicit
                  # values = [], meaning that no values are allowed

[[check-cfg.values]]
name = "feature"         # feature will be automatically set by cargo, but
                         # could still be extented
values = ["foo", "bar"]