libxfce4util can not be build under OpenSolaris (build 134) in the default mode. 
The reason for this is because xfce-miscutils.c includes varargs.h as a result of configure which sets HAVE_VARARGS_H to true.

On solaris platform, varargs.h defines va_start macro as resiving one parameter, and compilation stops at line 211. 
Can be very easily fixed by including stdarg.h instead of varargs.h, and the file already has support for it, all that is need is to add this to configure in order to produce correct config.h.