WIP: Add Centos 6 and Fedora 29 CMake build and test jobs to ci
requested to merge magjac/graphviz:add-centos6-and-fedora29-cmake-build-and-test-jobs-to-ci into master
By building and testing the oldest releases of each OS, we ensure that we don't use CMake constructs that are not available in them.
Edited by Magnus Jacobsson