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

[roymacdonald]

This is a good idea but just to keep in mind: addons_config.mk is used by the make filesystem, thus it needs to conform those standards but also have that in consideration so it those addons are handled properly both by PG and the make build system