Fix build with GCC 8 and yaml-cpp 0.6 (thanks Kevin Kofler)
    
    This adds patches that make the thing build with yaml-cpp 0.6
    and GCC 8, the latter submitted upstream (the former is a bit
    too hacky to go upstream as-is), and reverts the changes to try
    and build with a bundled yaml-cpp. Thanks to Kevin Kofler for
    helping to figure out both issues.