A define in addons_config.mk to indicate what platforms are supported

[ofTheo]

This way you can have a cross platform project where addons which are Linux only or Windows only can exist in the project's addons.make and if the addon isn't supported on a platform the PG doesn't add it.

We would prob make an empty define mean all for backwards compatibility but if it is defined it could be limited to just a few platforms eg for ofxAzureKinect

PLATFORMS=linux64,vs,msys2