stage 1 of replacing usage of various checks for the windows platform
with the compiler-defined '_WIN32' macro

Change-Id: Iece73abdee530937e0737190b1aa97a46cd3075f
Reviewed-on: https://gerrit.libreoffice.org/22390Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>