Add cargo-c

[yselkowitz]

The following packages use the cargo-c command to build C libraries from Rust code, and not in ways that easily lends itself to vendoring. So far we've been blocking cargo-c from ELN; should we just go ahead and accept it as a necessary build dependency:

• librsvg2
• rust-gst-plugin-gtk4

/cc @tpopela (pretty sure these are actually yours)

[tpopela]

If it is really needed, then we really can't do much about it as we really need librsvg2 in RHEL as well as snapshot (which requires rust-gst-plugin-gtk4). Probably cargo-c could be a good candidate to be a BUILDROOT only package.

[yselkowitz]

What I failed to notice is that cargo-c, through its (obvious) dependency on the cargo crate, pulls in libssh2, which is unwanted in RHEL. The workaround rust itself uses for this works for vendored dependencies, so either we need an eln branch to carry a similar patch, or modify rust-cargo itself to disable the ssh feature with a flag. (OTOH it seems only rust-cargo-c uses rust-cargo, so doing so shouldn't be too impactful.)