...and neither exclusively nor additionally as an environment variable (where
bootstrap variables include environment variables, see the @file comment in
include/rtl/bootstrap.h).  Note that LibLibreOffice_Impl::initialize
(desktop/source/lib/init.cxx) sets it as a bootstrap, not as an environment
variable.  (Though desktop/unx/source/start.c still only reads it as an
environment variable; could potentially check argv for
-env:SAL_USE_VCLPLUGIN=...)

Change-Id: I9315f585cdb4f0783181359e030686167c4e4625