Add a warning about the use of Debug or "" in CMAKE_BUILD_TYPE

Installation/CMakeLists.txt

@@ -378,6 +378,7 @@ set( CGAL_MODULES_REL_DIR cmake/modules )
 set( CGAL_MODULES_DIR     ${CGAL_INSTALLATION_PACKAGE_DIR}/${CGAL_MODULES_REL_DIR} )
 
 include(${CGAL_MODULES_DIR}/CGAL_Macros.cmake)
+include(${CGAL_MODULES_DIR}/CGAL_enable_end_of_configuration_hook.cmake)
 cgal_setup_module_path()
 
 message( STATUS "CGAL_REFERENCE_CACHE_DIR=${CGAL_REFERENCE_CACHE_DIR}" )

Installation/cmake/modules/CGALConfig_binary.cmake.in

@@ -190,3 +190,5 @@ if( CGAL_DEV_MODE OR RUNNING_CGAL_AUTO_TEST )
   # Nov. 2017). -- Laurent Rineau
   include(CGAL_SetupFlags)
 endif()
+
+include("${CGAL_MODULES_DIR}/CGAL_enable_end_of_configuration_hook.cmake")

Installation/cmake/modules/CGALConfig_install.cmake.in

@@ -165,3 +165,4 @@ if( CGAL_DEV_MODE OR RUNNING_CGAL_AUTO_TEST )
   # Nov. 2017). -- Laurent Rineau
   include(CGAL_SetupFlags)
 endif()
+include("${CGAL_MODULES_DIR}/CGAL_enable_end_of_configuration_hook.cmake")

Installation/cmake/modules/CGAL_enable_end_of_configuration_hook.cmake

@@ -0,0 +1,28 @@
+function(CGAL_run_at_the_end_of_configuration variable access value current_list_file stack)
+  if(value)
+    # Only do something at the end of the CMake process
+    return()
+  endif()
+  # Warn when CMAKE_BUILD_TYPE is empty or Debug
+  if(DEFINED CMAKE_BUILD_TYPE AND ( NOT CMAKE_BUILD_TYPE OR CMAKE_BUILD_TYPE STREQUAL "Debug") )
+    set(keyword WARNING)
+    if(RUNNING_CGAL_AUTO_TEST)
+      # No warning in the CMake test suite, but a status message
+      set(keyword )
+    endif()
+    if(NOT CGAL_DO_NOT_WARN_ABOUT_CMAKE_BUILD_TYPE)
+      message(${keyword} "\
+=======================================================================\n\
+CGAL performance notice:\n\
+The variable CMAKE_BUILD_TYPE is set to \"${CMAKE_BUILD_TYPE}\".  That \
+value should be used only for debugging. For performance reasons, you \
+should set CMAKE_BUILD_TYPE to \"Release\".\n\
+Set CGAL_DO_NOT_WARN_ABOUT_CMAKE_BUILD_TYPE to TRUE if you want to \
+disable this warning.\n\
+=======================================================================\
+")
+    endif()
+  endif()
+endfunction()
+
+variable_watch("CMAKE_CURRENT_LIST_DIR" CGAL_run_at_the_end_of_configuration)

Installation/lib/cmake/CGAL/CGALConfig.cmake

@@ -139,3 +139,5 @@ include(${CGAL_MODULES_DIR}/CGAL_Macros.cmake)
 cgal_setup_module_path()
 
 set(CGAL_USE_FILE ${CGAL_MODULES_DIR}/UseCGAL.cmake)
+
+include("${CGAL_MODULES_DIR}/CGAL_enable_end_of_configuration_hook.cmake")