Add checking that `cargo` exists when building gccrs

[CohenArthur]

Add checking for the existence of cargo when building gccrs, erroring out nice and early if it is not present - this will improve the user experience now that we rely on Rust components.

[tschwinge]

This is in context of #2822 "Start reusing rustc's format_args parser".

I'm actually curious: is it really erroring out, or is it degraded functionality?

[CohenArthur]

it's erroring out properly, but mjw suggested adding that check to cut out the compilation earlier in case cargo isn't present. it will make the experience better

[CohenArthur]

https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/Generic-Programs.html