Updates the existing Linux plugins to use 'make', using slightly modified
copies of the example_plugin Makefile. As part of that change, restructures
the Linux plugin sources to remove the src/ and include/... structure since
it was a legacy of older structures (C++ namespaces expressed via folder
name, direct inclusion of headers from the tree rather than the build output).

This moves the plugins a small step closer to what actual Linux plugin
development will look like (since to the extent that there is any build
logic in plugins themselves on Linux, the template would almost certainly
use Makefiles rather than GN).

This removes the last use of GN, so all the GN infrastructure is removed
from the project as well.

Fixes #392