Cmake fails when -DOGL_USE_EXTERNAL_GINKGO=On is set

[greole]

See title, building with cmake when setting -DOGL_USE_EXTERNAL_GINKGO=On . The relevant error message is as follows

CMake Error at /home/go/data/code/ginkgo/cmake/Modules/FindHWLOC.cmake:124 (add_library):
  add_library cannot create imported target "hwloc" because another target
  with the same name already exists.
Call Stack (most recent call first):
  /usr/local/lib/cmake/Ginkgo/GinkgoConfig.cmake:185 (find_package)
  CMakeLists.txt:27 (find_package)

[tcojean]

That's interesting, if I understand there is some mixed setup as well where the installed GinkgoConfig.cmake uses the module from the Ginkgo source? That should change with ginkgo-project/ginkgo#745 but I don't think that should fix the problem. I will fix it as part of this PR.