meson: Stop if cfi is enabled with system slirp

meson: Stop if cfi is enabled with system slirp

For CFI, we need to compile slirp as a static library together with qemu.
This is because we register slirp functions as callbacks for QEMU Timers.
When using a system-wide shared libslirp, the type information for the
callback is missing and the timer call produces a false positive with CFI.

With this patch, meson will stop if CFI is enabled with system-wide slirp.

In 6.1 we will introduce a new interface to slirp where the callback is
passed as an enum rather than a function pointer.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-Id: <20210304025939.9164-1-dbuono@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>