On systems where rustup is used, simple rpmbuild will use rustc from system,
but rustdoc will be taken from PATH which might be different from system.
Given this, doc-tests or anything related to rustdoc will fail with mis-mathching
rustc version.
Merges: https://pagure.io/fedora-rust/rust2rpm/pull-request/38
Since rustc doesn't allow `-g` and `-Cdebuginfo` options to mix, we need
to make sure we're consistent with the option crates may set in their
own `[profile] debug` settings. Cargo used to send `-g` for that, but
switched to `-Cdebuginfo` in version 0.17.0.
Fixes#32.
