[binary addons] a few cmake fixes

[Montellese]

Based on the comments in 068aafc I tried the binary addons buildsystem again and found some oddities. The first two commits are actual fixes and the other two are cosmetics.

I've only tested this on win32.

[FernetMenta]

Thanks! Tested on Linux and I was able to build two addons after having cleared cmake/addons/addons.
If this folder already has some content from previous builds, cmake fails silently. Means it just ignored the additional or different addons. Scenario:
I work on pvr.vdr.vnsi and built this addon. This folder was created: cmake/addons/addons/pvr.vdr.vnsi. Now I switch to inputstream.mpd and build this. the folder inputstream.mpd won't get created as long addons/addons is not empty. There is no error on the output of cmake.

[wsnipex]

Did you bootstrap the addon repo? I am pretty sure it has to do with that as it worked fine before when we still had the addon definitions in kodi src

[Montellese]

@wsnipex: This is a case that never worked (at least for me). The problem is that we run cmake to generate the buildsystem on the addons directory and after the buildsystem has been generated you change the content of the addons directory. But cmake sees that the buildsystem has already been generated and doesn't care about the changes that have been made to the addons directory.

The case where there are multiple addon definitions available but you only build one and then later on another one should still work (and works fine for me on win32).

[FernetMenta]

@Montellese can we merge this?

[Montellese]

jenkins build and merge