gst: Add better support for static plugins

gst: Add better support for static plugins

API: GST_PLUGIN_STATIC_DECLARE()
API: GST_PLUGIN_STATIC_REGISTER()

Based on a patch by Håvard Graff <havard.graff@tandberg.com>.

This now allows GST_PLUGIN_DEFINE() to create a static plugin if
GST_PLUGIN_BUILD_STATIC is defined. The resulting plugin can be
statically linked or dynamically linked during compilation but
can't be dynamically loaded during runtime.

Also adds GST_PLUGIN_STATIC_DECLARE() and GST_PLUGIN_STATIC_REGISTER(),
which allows to register a static linked plugin easily.
It is still required to manually register every single statically linked
plugin from inside the application as this can't be automated in a portable
way.

A new configure parameter --enable-static-plugins was added that allows
to build all plugins we build here as static plugins.

Fixes bug #667305.