meson: honor --enable-rbd if cc.links test fails

If the link test failed, compilation proceeded with RBD disabled,
even if --enable-rbd was used on the configure command line.
Fix that.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -690,15 +690,21 @@ if not get_option('rbd').auto() or have_block
   librbd = cc.find_library('rbd', has_headers: ['rbd/librbd.h'],
                            required: get_option('rbd'),
                            kwargs: static_kwargs)
-  if librados.found() and librbd.found() and cc.links('''
-    #include <stdio.h>
-    #include <rbd/librbd.h>
-    int main(void) {
-      rados_t cluster;
-      rados_create(&cluster, NULL);
-      return 0;
-    }''', dependencies: [librbd, librados])
-    rbd = declare_dependency(dependencies: [librbd, librados])
+  if librados.found() and librbd.found()
+    if cc.links('''
+      #include <stdio.h>
+      #include <rbd/librbd.h>
+      int main(void) {
+        rados_t cluster;
+        rados_create(&cluster, NULL);
+        return 0;
+      }''', dependencies: [librbd, librados])
+      rbd = declare_dependency(dependencies: [librbd, librados])
+    elif get_option('rbd').enabled()
+      error('could not link librados')
+    else
+      warning('could not link librados, disabling')
+    endif
   endif
 endif