A boilerplate project with testing, common libs for data structures, and pointers to useful resources like extra libs, would be really cool. :)
That said if I had to do something nowadays that I would use C for, I'd probably just pick up Rust.
I really am not sure if this is a joke or not, but almost every major library has a C interface. Almost all packages in your distro if you use linux have development versions that ship libraries that usually are linkable by C. In fact the wealth of libraries usable from C is usually noted as its main strength.
What exactly are you missing, and have you tried googling? The GNU organization and similar all produce libraries(like https://www.gnu.org/software/gsl/) that are mostly usable from C.
Anecdotally the types of libraries shippable with the OS represent a specific category of application which doesn't encompass many developers needs. (Although on the other side, github does not provide usable drivers for most operating systems... your mileage may very).
Further: https://github.com/oz123/awesome-c as another example of the many many C libraries you could find.
Many practical C projects seem to simply fork any dependencies they want to use and stick them in their own internal repo/build process.
Oh and it’s so poorly documented in practice that you have to cross reference it with codebases to see how the tooling is actually used in practice, then peel back all the weird quirks from those authors.
That’s the hard part.
Googling for a C library isn’t.