collectd: add write_prometheus plugin support

Add explicit support for the write_prometheus plugin, it requires the
libmicrohttpd and protobuf-c libraries.

[Peter: add note about where dependencies come from]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>