Possibility to override versions

[jstastny]

This enables running: make all IMAGEMAGICK_VERSION=7.0.9-20

We conditionally pass parameters like IMAGEMAGICK_VERSION=7.0.9-20 when invoking make inside Docker. The DOCKER_MAKE_PARAMS contains conditions to only have the parameters that are not empty. Otherwise we would pass empty variables like IMAGEMAGICK_VERSION= and the conditional parameter in Makefile_ImageMagick like IMAGEMAGICK_VERSION ?= 7.0.8-45 would not get applied.