Improved the isolation of windows headers.
Specifically, RGB macro is better handled now.

Change-Id: I0eeea16d0de9da3455810c80b0715f7b54ae8c3f
Reviewed-on: https://gerrit.libreoffice.org/20039Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>