meson: use pkg-config method to find dependencies

We do not need to ask cmake for the dependencies, so just use the
pkg-config mechanism.  Keep "auto" for SDL so that it tries using
sdl-config too.

The documentation is adjusted to use SDL2_image as the example,
rather than SDL which does not use the "pkg-config" method.

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