Revert "easylogging++: ensure it finds boost headers"

This reverts commit 9a95827ea7.

CMakeLists.txt

@@ -451,6 +451,7 @@ ExternalProject_Add(generate_translations_header
   CMAKE_ARGS -DLRELEASE_PATH=${LRELEASE_PATH}
   INSTALL_COMMAND cmake -E echo "")
 include_directories("${CMAKE_CURRENT_BINARY_DIR}/translations")
+add_subdirectory(external)
 
 # Final setup for libunbound
 include_directories(${UNBOUND_INCLUDE})
@@ -1023,5 +1024,3 @@ if (iwyu_tool_path AND PYTHONINTERP_FOUND)
     VERBATIM
   )
 endif()
-
-add_subdirectory(external)

external/easylogging++/CMakeLists.txt

@@ -40,7 +40,6 @@ add_library(easylogging
 
 include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
 include_directories("${CMAKE_CURRENT_BINARY_DIR}")
-include_directories(SYSTEM ${Boost_INCLUDE_DIRS})
 target_link_libraries(easylogging
   PRIVATE
     ${CMAKE_THREAD_LIBS_INIT}