configure.ac: Added disable switches for features dependending on external libs.

This makes packagers' work easier, because can choose whether they want to introduce new dependency for
their package.
configure: enable-*=yes options make features mandatory.

If a feature is switched on by --enable-feature=yes, then if a
dependency for this feature is missing, configure fails. If it is
switched on by --enable-feature=auto, then if a dependency is missing,
the feature is switched off.
Merge pull request #49 from talien/f/disable-features

configure.ac: Added disable switches for features dependending on extern...