We need to redefine CFLAGS to drop missing-prototypes as warning since
third-party code such as wasmtime fails to build from source when
building a debian package. This happens only for packages because we
use DPKG_EXPORT_BUILDFLAGS=1 propagating build flags to the environment,
so cargo build picks it up as well. Since we have no control over
third-party code, the easiest solution is to disable this warning.
