message: COMP: Support multiple find_package(Gengetopt)
  The following should not produce any error:
  ```
  cmake_minimum_required(VERSION 3.15)
  project(testFindGengetopt)
  list(APPEND CMAKE_MODULE_PATH "${RTK_SOURCE_DIR}/cmake")
  include("${RTK_SOURCE_DIR}/cmake/rtkCompilerFlags.cmake")
  find_package(Gengetopt REQUIRED)
  find_package(Gengetopt REQUIRED)
  ```
diff: -
  # Attempt to find gengetopt. If not found, compile it.
  +if (NOT TARGET gengetopt)
  find_program(GENGETOPT gengetopt)
  if ((GENGETOPT STREQUAL "GENGETOPT-NOTFOUND") OR (GENGETOPT STREQUAL ""))
  get_filename_component(CLITK_CMAKE_DIR ${CMAKE_CURRENT_LIST_FILE} PATH)
  @@ -13,6 +13,7 @@ else()
  message(FATAL_ERROR "No gengetopt executable found at the specified location")
  endif()
  endif()
  +endif()
  # Create a cmake script to cat a list of files
  file(WRITE "${CMAKE_BINARY_DIR}/cat.cmake" "
  