auto-detect the presence of a native gmake on windows

Change-Id: Ie1c7e022a2e1713a111ebd2db9db866e09f4f4f4
Reviewed-on: https://gerrit.libreoffice.org/10120
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>