cc 1.0.87 produces tons of warnings on cargo check

[ahlinc]

After update to the latest cc 1.0.87 I see tons of warnings for every dependency crate that uses cc in its build.rs.
Warnings looks like:

warning: some-dependency-crate@0.0.1: running: "cc" "--version"
warning: some-dependency-crate@0.0.1: exit status: 0
warning: some-dependency-crate@0.0.1: running: "cc" "--version"
warning: some-dependency-crate@0.0.1: exit status: 0
warning: some-dependency-crate@0.0.1: running: "cc" "--version"
warning: some-dependency-crate@0.0.1: exit status: 0
warning: some-dependency-crate@0.0.1: running: "cc" "--version"
warning: some-dependency-crate@0.0.1: exit status: 0
warning: some-dependency-crate@0.0.1: running: "ar" "s" "/home/user/some-code/target/debug/build/some-dependency-crate-b229f6f8a8f92087/out/libfoo.a"
warning: some-dependency-crate@0.0.1: exit status: 0

It seems like that some debug logging was overlooked and now the cc crate reports all sub command exit statuses as cargo warnings.

[NobodyXu]

It's #960 where I suggest to turn them into warnings, I think we need to add a new debugging printing that is enabled by default (but without cargo:warning= prefix so it's not shown by default), that can be disabled.

[NobodyXu]

cc @amaanq