make XMESSAGE a variable and use it consistently
make XMESSAGE a variable and use it consistently

Previously, gxmessage was getting substituted in one place and not
another.

naimdjon/fluxbox,luebking/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,luebking/fluxbox,lukoko/fluxbox,Arkq/fluxbox,lukoko/fluxbox,luebking/fluxbox,lukoko/fluxbox,olivergondza/fluxbox,Arkq/fluxbox,lukoko/fluxbox,Arkq/fluxbox,olivergondza/fluxbox,naimdjon/fluxbox,luebking/fluxbox,Arkq/fluxbox,naimdjon/fluxbox,luebking/fluxbox,olivergondza/fluxbox,naimdjon/fluxbox,lukoko/fluxbox,naimdjon/fluxbox,olivergondza/fluxbox