message: add check if .git is a directory
  Fixes CMake error in string(SUBSTRING) if OSPRay is a git submodule
diff: @@ -20,7 +20,8 @@ set(OSPRAY_VERSION_PATCH 5)
  set(OSPRAY_SOVERSION 0)
  set(OSPRAY_VERSION_GITHASH 0)
  set(OSPRAY_VERSION_NOTE "")
  -if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/.git)
  +if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/.git AND
  + IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/.git)
  find_package(Git)
  if (GIT_FOUND)
  execute_process(
  