slirp: Convert Makefile bits to meson bits

slirp: Convert Makefile bits to meson bits

SLIRP uses Meson so it could become a subproject in the future,
but our choice of configure options is not yet supported in Meson
(https://github.com/mesonbuild/meson/pull/7740).

For now, build the library via the main meson.build just like for
capstone.

This improves the current state of affairs in that we will re-link
the qemu executables against a changed libslirp.a, which we wouldn't
do before-hand.

Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>