Where is version in so's set? Seems to have disappeared w/gcc upgrade?

[L A Walsh]

I compiled & installed the gcc suite to get the 6.x toolset (only had 4.x
available before).

A side effect of this has cropped up in various libs generated
by the new toolset that other tools link to:

/home/opt/Qt/5.8/gcc_64/plugins/platforms/libqxcb.so: 
/usr/lib64/libQt5Core.so.5: no version information available (required 
by /home/opt/Qt/5.8/gcc_64/plugins/platforms/libqxcb.so)

Why did this disappear w/new gcc?   Did I leave off some
switch in gcc-config or in compiling or linking the libs?

Why is an internal version number needed?  Wasn't the
".so" extension supposed to allow for library versioning
along the lines of:

foo.so.4.3
foo.so.5.8
foo.so.4 => foo.so.4.3
foo.so.5 => foo.so.5.8
foo.so => foo.so.5

(not that such is used very often on linux for some reason).

Thanks much!
Linda