[lmi] Is '-Wno-unused-variable' still needed?

[Greg Chicares]

While inspecting some recent changes, I saw that 'configure.ac' today
contains:

    dnl This warning is now only given for 2 variables in ledger_evaluator.cpp
    dnl and should be fixed there and reenabled.
    LMI_CXX_ADD_IF_SUPPORTED(-Wno-unused-variable)

It looks like you added those "dnl" comments in commit 5112db4dcfcfe,
about two weeks ago, so I suppose you've seen unused-variable warnings
in an autotools build. But I see no such warnings in builds using
lmi's makefiles. Why would there be a difference? AFAICT, '-Wall'
should enable '-Wunused-variable', and I'm not turning it off.

(We do use '-Wno-unused-variable' when building some xmlsoft.org
libraries, presumably for good reason; but I don't think that can
affect 'ledger_evaluator.cpp'.)