Turns out IS_DIRECTORY needs an absolute path.

vendor/CMakeLists.txt

@@ -5,21 +5,21 @@
 # Most of these libraries are used for running comparison benchmarks against
 # other logging libraries.
 
-if(IS_DIRECTORY zf_log)
+if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/zf_log")
     add_subdirectory(zf_log)
 endif()
 
-if(IS_DIRECTORY glog)
+if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/glog")
     add_subdirectory(glog)
 endif()
 
-if(IS_DIRECTORY easyloggingpp)
+if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/easyloggingpp")
     add_library(easylogging INTERFACE)
     set(SPDLOG_VENDORED_EASYLOGGING_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/easyloggingpp" CACHE INTERNAL "" FORCE)
     target_include_directories(easylogging INTERFACE "${SPDLOG_VENDORED_EASYLOGGING_ROOT}/src")
 endif()
 
-if(IS_DIRECTORY g3log)
+if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/g3log")
     add_subdirectory(g3log)
 endif()