...in various places, which is gone by default at least from recent libc++ in
C++17 mode.  So bring it back there for now, until Boost is fixed.

Change-Id: I0b3128d114ab2b5693a84bdc7d2b9b9b9b5c3c76
Reviewed-on: https://gerrit.libreoffice.org/45910Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>