fix to allow building on both Ubuntu 16.04 and 18.04.

[matjam]

As promised in mosra/magnum#248, here is a PR that will allow builds to work for both Ubuntu 18.04 and 16.04. Note that some of the changes are also to allow the build to work correctly in Launchpad, as it builds from a clean chroot every time dependencies may not be available.

[codecov-io]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   19.24%   19.24%           
=======================================
  Files          35       35           
  Lines        1408     1408           
=======================================
  Hits          271      271           
  Misses       1137     1137

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[mosra]

Merged in 349675f, though without the explicit Corrade dependency (as we discussed on Gitter) -- if it turns out that the dependency is really needed on Launchpad, we can always add it later.

Thank you a lot! :)