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>
diff --git a/net/meson.build b/net/meson.build
index 6c2ec47..1c7e3a3 100644
--- a/net/meson.build
+++ b/net/meson.build
@@ -18,7 +18,7 @@
 ))
 
 softmmu_ss.add(when: 'CONFIG_L2TPV3', if_true: files('l2tpv3.c'))
-softmmu_ss.add(when: ['CONFIG_SLIRP', slirp], if_true: files('slirp.c'))
+softmmu_ss.add(when: slirp, if_true: files('slirp.c'))
 softmmu_ss.add(when: ['CONFIG_VDE', vde], if_true: files('vde.c'))
 softmmu_ss.add(when: 'CONFIG_NETMAP', if_true: files('netmap.c'))
 vhost_user_ss = ss.source_set()