examples: skip build when missing dependencies

examples: skip build when missing dependencies

Trying to disable the vhost library, meson will complain it can't build
the vhost* and vdpa examples when passing -Dexamples=all.

-Dexamples=all skips examples if the example itself announces it can't
be built (for external dependencies, internal dependencies and other
reasons).
Since examples/meson.build will evaluate the internal dependencies
in any case, let's move the check there and resolve the issue for
optional internal libraries.

Fixes: 0bf583222297 ("lib: allow disabling optional libraries")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>