Merge pull request #4257 from DonLakeFlyer/PluginBuildSystem

Allows QGC plugins to be optional or custom

From #4256 

> This allows you to create custom builds of QGC which may or may not contain all of the plugins. It also allows you to create a custom build where you can re-use the PX4 or APM plugin code as the base class for you own custom vehicle plugin and then expose only that to the system.