Avoid gcc/10.1.0 bug by moving some configuration into a namespace

Avoid gcc/10.1.0 bug by moving some configuration into a namespace

Eventually there is intention to move all of cvc5::Configuration into
cvc5::configuration, this is a first step that also has the nice side-effect of
letting it build under an otherwise-buggy version of gcc/10.1.0 that was
causing issues.

Signed-off-by: Matthew Sotoudeh <sotoudeh@stanford.edu>