making the build between Simgear and Flightgear consistent.
- Fix an issue with lib/lib64 path between CentOS/RedHat distributions
and Debian derived distributions.
- Fix a compilation issue with HLA in the Flightgear code.
This is necessary on macOS where DBus is typically installed using
Homebrew, and hence we need additional linker flags to find the
library. Using the PkgConfig::DBUS imported target handles all this
transparently, but then we need to define an imported target for
Windows to work.