configure: Improve requesting specific features

configure: Improve requesting specific features

Before this patch trying to enable only https by issuing

    --disable-protocols --enable-protocol=https --enable-gnutls

does not enable https, and

    --disable-all --enable-protocols

does not enable libavformat and the protocols component.

Now the default-enabled options are set after the explicitly
disabled/enabled options are evaluated.

If an explicitly enabled option cannot be enabled configure
will fail printing an error message.