_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING turned into a global
-D..., same as existing
_SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING.

Change-Id: Ifefdc930a1d3ecf5366523dea509bfaca1f4e002
Reviewed-on: https://gerrit.libreoffice.org/55188Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>