This extends the approach used for debian-based packages in 3f805bc64e28
to rpm as well.  Notable change for both deb and rpm packaging is to use
CFLAGS as defined in the build/Makefile, and not pass them from the
environment which might not be there (as is the case for rpm).

While at it, stop passing CFLAGS in the install phase, as it should no
longer invoke builds (see d54af163c46b).

The rpm part was overlooked in 7a6405566c0, since testing was not done
on the platforms where problem manifested itself, notably Amazon Linux
2023 and Fedora 38+.

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.