devel/cargo-c: unbreak WITH_DEBUG in consumers after c9724410bb38

error: The argument '--debug <debug>' requires a value but none was supplied

Reported by: mizhka
Regressed by: https://github.com/lu-zero/cargo-c/commit/32f7e107b17c

(cherry picked from commit 577b194fe1da55b53b73d2f18caf20605d6f8ce8)