configure.ac: Ensure libomxil-bellagio exists before invoking pkg-config.

configure.ac: Ensure libomxil-bellagio exists before invoking pkg-config.

I was already tired of seeing the message

    Package libomxil-bellagio was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libomxil-bellagio.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libomxil-bellagio' found

on every configure, but I just got a distro bug reported where the user
was confused by this message and thought it indicated a bug.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>