configure.in: Let configure bail out on missing dependencies

configure.in: Let configure bail out on missing dependencies

The patch below adds check in AC_PLUGIN() that verifies if the
dependencies are met for any enabled plugin.

In case some dependency is missing (default is disabled and enabled
is selected) configure will fail after dumping summary so all
dependency issues are visible in a single configure pass.

A later addition would be to list requirements (dependencies, OS/Kernel
restrictions) for all plugins in an easily accessible location (e.g.
INSTALL file)

Signed-off-by: Florian Forster <octo@huhu.verplant.org>