Fixing build failure with Inventor and SoXt

[auto-07p]

Fixing build failure with Inventor and SoXt

[bartoldeman]

Thanks for the fixes! plaut04 has not been deprecated, but I haven't tested the soxt/coin version in many years, and using the SGI Open Inventor Xt Library I can't even remember, instead using Coin and SoQt with Qt4.

Now I realize that Qt4 is getting old but as you say it's more work to port to Qt5 (it would be most welcome but haven't had time to do that yet). As e.g Ubuntu 18.04 LTS, Debian 10, and Fedora still ship Qt4 and Coin with libsoqt for Qt4 it still works in many cases but not for much longer I see, and Inventor is a nice stopgap alternative with libmotif freely available.

[bartoldeman]

I just pushed a change to the configure script to detect Qt5 and try to use SoQt with it. I've only tested this with Debian's libsoqt520-dev package so far. Perhaps this works for you as well?

I was surprised no source code changes were necessary though there do seem to be some runtime warnings. The SoQt version certainly looks prettier.