Build: Let the users override the symbol versioning variant.

Build: Let the users override the symbol versioning variant.

There are cases when the users want to decide themselves whether
they want to have the generic (even on GNU/Linux) or the linux
(even if we do not recommend that) symbol versioning variant.
The former might be needed to circumvent compiler issues (i.e.
the compiler does not support all features that are required
for the linux versioning), the latter might help in overriding
the assumptions made in the configure script.