It seems like recent changes to ImageMagick have completely broken this, giving Error 127 (aka can't even load the plugin) in the Rainmeter log.
I have upgraded my ImageMagick version from 7.0.10 to 7.1.1 and the problem above is constant.
The only thing I can say is that MagickMeter.h in this plugin is just a modified ImageMagick/MagickCore/magick-type.h, and all that's been done to that file is fix compile warnings (the same errors appear when trying to build MagickMeter too), so it must mean other structural changes inside Magick++ between 7.0.10 and 7.1.1 have most likely broken this.