Draft: pass CC and CFLAGS to dependent crates
This PR adds passing of RIOT's CC and CFLAGS values to dependent crates.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key for how "links" is supposed to work.
Here, we don't really link to a native "riot-sys" library, but setting
links = "riot-sys" still makes cargo pass on key/value pairs specified by build.rs printing "cargo:KEY=value".
(will open a PR to riot-wrappers making use of this shortly).
Corresponding riot-wrappers MR.
This change shouldn't affect the current way of building. The riot-wrappers change would require this riot-sys change if merged. So maybe a version increase is necessary here.