#27 makefilet-download-ondemand.mk will happily load older versions of makefilet

One of the surprises of system installations of makefilet is that it might load an older version of itself, even when specifying a more recent versions in the `makefilet-download-ondemand.mk`. Though the variable name `MAKEFILET_DOWNLOAD_VERSION` does reflect that the version is only used in case makefilet is downloaded at runtime, loading old versions of itself might easily break a build process or sometimes worse introduces very subtle differences in how a build is processed. These problems are often hard to spot. I think a makefilet system-installation should 1. either check if the requested version in `MAKEFILET_DOWNLOAD_VERSION` is more recent and refuse to load any further configuration 2. or should output visually distinctive warnings about version mismatches throughout the build process and allow system-loading to be disabled entirely by means of configuration

fixed in f423588b38f2bcbdbc3606bf1561abb30a5b0350