ExternalProject: Add CMAKE_CACHE_DEFAULT_ARGS arguments

This argument allows to set default arguments that are written in the
initial cache file, but that are not forced, and therefore allows the
user to change these values later (CMAKE_ARGS and CMAKE_CACHE_ARGS
always overwrite the values).

Also add some documentation to explain the differences between these 3
arguments.

Extend ExternalProjectLocal test to cover CMAKE_CACHE_DEFAULT_ARGS.

Tests/ExternalProjectLocal/CMakeLists.txt

@@ -66,6 +66,7 @@ if(can_build_tutorial_step5)
   ExternalProject_Add(${proj}
     URL "${CMAKE_CURRENT_SOURCE_DIR}/../Tutorial/Step5"
     CMAKE_ARGS -DCMAKE_INSTALL_PREFIX:PATH=<INSTALL_DIR> -G ${CMAKE_GENERATOR} <SOURCE_DIR>
+    CMAKE_CACHE_DEFAULT_ARGS -DUSE_MYMATH:BOOL=OFF
     TEST_AFTER_INSTALL 1
     LOG_TEST 1
   )