spdlog can use the bundled header-only fmt lib or a shared lib. However,
compatibility to the external fmt lib is somewhat fragile. But when
using e. g. the package from the linux distribution we cannot decide to
use the bundled fmt lib (it is not bundled in the package). With this
commit, we just test whether logging custom types work with a
preinstalled spdlog. If it works, we use it. If it does not work or if
it is not even installed, we build spdlog using the bundled fmt lib from
git.