Installing files out of prefix

[Jasper Huijsmans]

Hi,

I just noticed that the reason `make distcheck' fails for the panel is
that the mcs plugins are installed out of prefix, in the prefix for mcs
manager to be exact. 

Wouldn't it be better to look for a MCS_PLUGIN_PATH (or a
XFCE4_PANEL_PLUGIN_PATH for panel plugins) variable that can be used by
people who install packages in different locations. This is common
behaviour for other software, isn't it?

	Jasper