net: fix build when libbpf is disabled, but libxdp is enabled

The net/af-xdp.c code is enabled when the libxdp library is present,
however, it also has direct API calls to bpf_xdp_query_id &
bpf_xdp_detach which are provided by the libbpf library.

As a result if building with --disable-libbpf, but libxdp gets
auto-detected, we'll fail to link QEMU

  /usr/bin/ld: libcommon.a.p/net_af-xdp.c.o: undefined reference to symbol 'bpf_xdp_query_id@@LIBBPF_0.7.0'

There are two bugs here

 * Since we have direct libbpf API calls, when building
   net/af-xdp.c, we must tell meson that libbpf is a
   dependancy, so that we directly link to it, rather
   than relying on indirect linkage.

 * When must skip probing for libxdp at all, when libbpf
   is not found, raising an error if --enable-libxdp was
   given explicitly.

Fixes: cb039ef3d9e3112da01e1ecd9b136ac9809ef733
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

meson.build

@@ -2164,8 +2164,14 @@ endif
 # libxdp
 libxdp = not_found
 if not get_option('af_xdp').auto() or have_system
-    libxdp = dependency('libxdp', required: get_option('af_xdp'),
-                        version: '>=1.4.0', method: 'pkg-config')
+    if libbpf.found()
+        libxdp = dependency('libxdp', required: get_option('af_xdp'),
+                            version: '>=1.4.0', method: 'pkg-config')
+    else
+        if get_option('af_xdp').enabled()
+            error('libxdp requested, but libbpf is not available')
+        endif
+    endif
 endif
 
 # libdw

net/meson.build

@@ -39,7 +39,7 @@ if have_netmap
   system_ss.add(files('netmap.c'))
 endif
 
-system_ss.add(when: libxdp, if_true: files('af-xdp.c'))
+system_ss.add(when: [libxdp, libbpf], if_true: files('af-xdp.c'))
 
 if have_vhost_net_user
   system_ss.add(when: 'CONFIG_VIRTIO_NET', if_true: files('vhost-user.c'), if_false: files('vhost-user-stub.c'))