This module is similar to ../ImageMagick, except that instead of taking
the pre-compiled ImageMagick libraries from the build machine and only
compiling convert.c, here we fetch the ImageMagick source code and compile
it especially for OSv.

We compile "convert" as a PIE (trying to compile it as a shared object was
very difficult because of the convoluted buildtools layers used to build
ImageMagick), and we disable OpenMP in the build because of the yet-unfixed
problems in using it in OSv (see OSv issue #590).