Add ability to build static GST plugins

Note that this only works for 1.0 plugin.

https://bugs.freedesktop.org/show_bug.cgi?id=85929
tests: Fix a memory leak in test-send-recv

If n_messages ≠ n_valid_messages.
tests: Use G_SOURCE_[CONTINUE|REMOVE] instead of booleans

Makes the test code a little clearer. Introduces no functional changes.
agent: Add debug output for lifetime of Components and Streams
nicesink: unblock pending write on component change

When a nicesink switches to a different component, we should unblock any
pending write waiting in gst_nice_sink_render() for writable_cond being
signalled because if the new transport already is writable, we'll never
get notified about the condition and thus nicesink will wait forever.

That way, either the pending data are immediately sent or the nicesink
blocks again until the transport becomes writable.
Remove duplicate ips from nice_interfaces
OpenVPN creates a tun interface having the same IP assigned twice like:

9: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc ...100
    link/none
    inet 10.222.111.10 peer 10.222.111.9/32 brd 10.222.111.10 scope global tun0
       valid_lft forever preferred_lft forever
    inet 10.222.111.10/32 brd 10.222.111.10 scope global tun0
       valid_lft forever preferred_lft forever

This interface would generate two identical host candidates with the same value
of foundation.
TEMP: Revert "agent: Avoid leaking UPnP mappings between streams"

This reverts commit 375cde8a652f8e367cf875bcb718a6d1abcfb254.

We don't have gupnp-igd 0.2.4 in the repository for this change to compile.