Introduce gb_*_set_external_code

...to harmonize the mechanisms to not define LIBO_INTERNAL_ONLY for extension
code and CppunitTest_odk_checkapi.  (It also needs to revert any -Wundef, as the
config_host/config_*.h files will not be included, so the HAVE_* macros will be
undefined.)

This obsoletes the need for RTL_DISABLE_FAST_STRING.

Change-Id: If5eacba80c349efb90762aade8f2ea6d2db7e314