Created attachment 4580 Patch known to work on Solaris Like the summary says, the name "sun" is reserved on a Solaris system, and code attempting to use it for variable names fails to compile in strange ways. See line 894 of panel-plugin/weather.c gchar *fog, *cloudiness, *sun, *value; simplest fix is to replace "sun" by something similar, eg "sunny".
Fixed in http://git.xfce.org/panel-plugins/xfce4-weather-plugin/commit/?id=6a395f4829d7adbffa75e6abf62b0fa0df6e9e81 I renamed it to "sunval". Thanks for reporting.