if(NOT PREPROC_IS_EXTERNAL_PROJECT)
  set(PREPROC_PREFIX "" CACHE PATH "The path to the prefix of an libpreproc installation")
  set(PREPROC_INCLUDE_DIR "" CACHE PATH "The path to the headers of an libpreproc installation")
  set(PREPROC_LIBRARY_DIR "" CACHE PATH "The path to the library of an libpreproc installation")

  set(_search_paths "")
  if(PREPROC_INCLUDE_DIR AND EXISTS ${PREPROC_INCLUDE_DIR})
    list(APPEND _search_paths ${PREPROC_INCLUDE_DIR})
  endif()
  if(PREPROC_PREFIX AND EXISTS ${PREPROC_PREFIX})
    list(APPEND _search_paths "${PREPROC_PREFIX}/include")
  endif()
  find_path(
    PREPROC_INCLUDE_DIRS
    NAMES preproc/mark.hpp
    PATHS ${_search_paths})

  set(_search_paths "")
  if(PREPROC_LIBRARY_DIR AND EXISTS ${PREPROC_LIBRARY_DIR})
    list(APPEND _search_paths ${PREPROC_LIBRARY_DIR})
  endif()
  if(PREPROC_PREFIX AND EXISTS ${PREPROC_PREFIX})
    list(APPEND _search_paths "${PREPROC_PREFIX}/lib")
  endif()
  find_library(
    PREPROC_LIBRARIES
    NAMES fsp
    PATHS ${_search_paths})
  unset(_search_paths)

  if(PREPROC_INCLUDE_DIRS AND PREPROC_LIBRARIES)
    set(PREPROC_FOUND TRUE)
  endif()

  if(PREPROC_FOUND)
    if(NOT PREPROC_FIND_QUIETLY)
      message(STATUS "Found libpreproc")
    endif()
    set(HAVE_PREPROC 1)
  elseif(PREPROC_FOUND)
    if(PREPROC_FIND_REQUIRED)
      message(FATAL_ERROR "Could not find libpreproc")
    endif()
  endif()
endif()