Support compilation with libboost version 1.54
Support compilation with libboost version 1.54

The configure.ac script allows even lower versions of boost to be used.
However, in cashaddrenc_tests.cpp, there is the assumption that
boost::variant has an operator!=(..) defined, though it has not.
See e.g.:

https://www.boost.org/doc/libs/1_54_0/doc/html/boost/variant.html

This works around the missing operator by using operator==(..) instead.

It also makes two variables const, as they can be made const.

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