Replace ../netgen-4.9.13/nglib/ng_vol.cpp with the version from this directory.
Then do a 'make' there.
A different netgen version might work. If not you may do a diff to the
original ng_vol (ng_vol.cpp_orig) and apply the changes to your ng_vol. 
Make sure that the resulting prg 'ng_vol' will be found by the system in any
folder you want to use with cgx.
It allows the specification of a element target size.
